Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2015	Mar. 01, 2016	Jun. 30, 2015
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	FY
Trading Symbol	AKBA
Entity Registrant Name	Akebia Therapeutics, Inc.
Entity Central Index Key	0001517022
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		37,942,946
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 259,927,754

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 49,778	$ 32,780
Available for sale securities	88,676	76,138
Prepaid expenses and other current assets	2,563	1,562
Total current assets	141,017	110,480
Property and equipment, net	540	210
Deferred offering costs	102
Other assets	1,281	305
Total assets	142,940	110,995
Current liabilities:
Accounts payable	2,313	2,021
Accrued expenses	9,555	4,864
Total current liabilities	11,868	6,885
Other liabilities	74	32
Total liabilities	$ 11,942	$ 6,917
Commitments and contingencies (Note 8)
Stockholders' equity:
Preferred stock $0.00001 par value, 25,000,000 shares authorized at December 31, 2015 and 2014; 0 shares issued and outstanding at December 31, 2015 and 2014
Common stock: $0.00001 par value; 175,000,000 shares authorized at December 31, 2015 and 2014; 30,662,218 and 20,370,624 shares issued and outstanding at December 31, 2015 and 2014, respectively	$ 0	$ 0
Additional paid-in capital	292,783	204,969
Treasury stock, at cost, 8,463 shares	(162)	(162)
Accumulated other comprehensive loss	(234)	(56)
Accumulated deficit	(161,389)	(100,673)
Total stockholders' equity	130,998	104,078
Total liabilities and stockholders' equity	$ 142,940	$ 110,995

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2015	Dec. 31, 2014
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.00001	$ 0.00001
Preferred stock, shares authorized	25,000,000	25,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	175,000,000	175,000,000
Common stock, shares issued	30,662,218	20,370,624
Common stock, shares outstanding	30,662,218	20,370,624
Treasury Stock, shares	8,463	8,463

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating expenses:
Research and development	$ 43,016	$ 23,263	$ 8,902
General and administrative	18,497	14,677	7,031
Total operating expenses	61,513	37,940	15,933
Operating loss	(61,513)	(37,940)	(15,933)
Other income (expense):
Interest income (expense), net	510	206	(704)
Extinguishment of debt and other liabilities			2,420
Other income	287	700	1,050
Net loss	(60,716)	(37,034)	(13,167)
Reconciliation of net loss to net loss applicable to common stockholders:
Net loss	(60,716)	(37,034)	(13,167)
Accretion on preferred stock		(86,899)	(55,886)
Net loss applicable to common stockholders	$ (60,716)	$ (123,933)	$ (69,053)
Net loss per share applicable to common stockholders—basic and diluted	$ (2.29)	$ (8.04)	$ (126.94)
Weighted-average number of common shares used in net loss per share applicable to common stockholders—basic and diluted	26,469,170	15,406,386	544,002
Comprehensive loss:
Net loss	$ (60,716)	$ (37,034)	$ (13,167)
Other comprehensive loss - unrealized loss on securities	(234)	(56)
Comprehensive loss	$ (60,950)	$ (37,090)	$ (13,167)

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Treasury Stock	Unrealized Gain/Loss	Accumulated Deficit	IPO	IPO Additional Paid-In Capital	Series A redeemable convertible preferred stock	Series B redeemable convertible preferred stock	Series C redeemable convertible preferred stock	2012 Series X Redeemable Convertible Preferred Stock
Balance at Dec. 31, 2012	$ (59,588)					$ (59,588)
Balance, shares at Dec. 31, 2012		615,757
Balance at Dec. 31, 2012									$ 37,092	$ 19,816
Balance, shares at Dec. 31, 2012									734,538	1,287,525
Issuance of restricted common stock, shares		583,126
Issuance of common stock, net of issuance costs, shares		176,717
Exercise of stock options, shares		14,357
Exercise of stock options	5		$ 5
Forfeitures of restricted common stock, shares		(6,612)
Reclassification of 2012 Series X preferred stock upon modification												$ 2,486
Reclassification of 2012 Series X preferred stock upon modification, shares												25,000
Sale of preferred stock, net of issuance cost											$ 40,088	$ 2,458
Sale of preferred stock, net of issuance cost, shares											2,945,742	25,000
Conversion of preferred stock, shares											357,143	(50,000)
Conversion of preferred stock											$ 4,944	$ (4,944)
Accretion of preferred stock to redemption value									$ 2,275	$ 1,441	52,170
Accretion of preferred stock to redemption value	(55,886)		(1,569)			(54,317)
Share-based compensation expense	1,564		1,564
Net income (loss)	(13,167)					(13,167)
Balance at Dec. 31, 2013	(127,072)					(127,072)
Balance, shares at Dec. 31, 2013		1,383,345
Balance at Dec. 31, 2013									$ 39,367	$ 21,257	$ 97,203
Balance, shares at Dec. 31, 2013									734,538	1,287,525	3,302,885
Issuance of restricted common stock, shares		56,000
Issuance of common stock, net of issuance costs	114,954		114,954
Issuance of common stock, net of issuance costs, shares		6,762,000
Receipt of payment on promissory notes issued in exchange for shares of common stock	237		237
Exercise of stock options, shares		116,394
Exercise of stock options	89		89
Forfeitures of restricted common stock, shares		(53,835)
Conversion of preferred stock, shares		12,115,183							(734,538)	(1,287,525)	(3,302,885)
Conversion of preferred stock	244,727		180,057			64,670			$ (74,188)	$ (45,514)	$ (125,025)
Accretion of preferred stock to redemption value									$ 34,821	$ 24,257	$ 27,822
Accretion of preferred stock to redemption value	(86,900)		(85,663)			(1,237)
Share-based compensation expense	6,010		6,010
Deferred offering costs- IPO							$ (10,715)	$ (10,715)
Unrealized gain/loss	(56)				$ (56)
Treasury shares purchased, not retired( 2,553)	(94)			$ (94)
Treasury shares purchased, not retired in shares		(2,553)
Treasury shares purchased, retired (5,910)	(68)			(68)
Treasury shares purchased, retired in shares		(5,910)
Net income (loss)	(37,034)					(37,034)
Balance at Dec. 31, 2014	$ 104,078		204,969	(162)	(56)	(100,673)
Balance, shares at Dec. 31, 2014	20,370,624	20,370,624
Issuance of common stock, net of issuance costs	$ 82,750		82,750
Issuance of common stock, net of issuance costs, shares		10,083,070
Exercise of stock options, shares	218,674	218,674
Exercise of stock options	$ 130		130
Proceeds from sale of stock under employee stock purchase plan	220		220
Proceeds from sale of stock under employee stock purchase plan, shares		25,903
Forfeitures of restricted common stock, shares		(36,053)
Share-based compensation expense	4,714		4,714
Unrealized gain/loss	(178)				(178)
Net income (loss)	(60,716)					(60,716)
Balance at Dec. 31, 2015	$ 130,998		$ 292,783	$ (162)	$ (234)	$ (161,389)
Balance, shares at Dec. 31, 2015	30,662,218	30,662,218

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities:
Net loss	$ (60,716)	$ (37,034)	$ (13,167)
Adjustments to reconcile net loss to net cash used in operating activities:
Gain on extinguishment of debt and other liabilities			(2,420)
Depreciation expense	96	49	1
Amortization of debt issuance costs			8
Amortization of premium/discount on investments	558	268	752
Stock-based compensation expense	4,714	6,010	1,564
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,001)	(686)	(217)
Other assets	(976)	(179)	(125)
Accounts payable	231	1,307	296
Accrued expenses	4,646	2,754	1,976
Other liabilities	41	28
Net cash used in operating activities	(52,407)	(27,483)	(11,332)
Investing activities:
Purchase of equipment	(414)	(229)	(19)
Proceeds from maturities of available for sale securities	63,901	12,585	1,990
Purchases of available for sale securities	(77,175)	(77,708)	(13,396)
Net cash used in investing activities	(13,688)	(65,352)	(11,425)
Financing activities:
Proceeds from the issuance of redeemable convertible preferred stock, net of issuance costs			40,047
Proceeds from the issuance of common stock, net of issuance costs	82,750	104,328	5
Proceeds from sale of stock under employee stock purchase plan	220
Proceeds from the exercise of stock options	130
Repurchase of treasury stock		(162)
Payments received on promissory notes issued in exchange for shares of common stock		237
Payments on capital lease obligations	(7)	(3)
Net cash provided by financing activities	83,093	104,400	42,331
Increase in cash and cash equivalents	16,998	11,565	19,574
Cash and cash equivalents at beginning of period	32,780	21,215	1,641
Cash and cash equivalents at end of period	49,778	32,780	21,215
Non-cash financing activities:
Conversion of series A, series B and series C preferred stock into common stock		244,727
Accretion of preferred stock to redemption value		$ 86,899	55,886
Unpaid initial public offering issuance costs			857
Unpaid follow-on offering costs	102
Assets acquired under capital lease	$ 12		12
Conversion of 2012 Series X preferred stock into Series C preferred stock			4,944
Series X Preferred Stock
Financing activities:
Proceeds from the issuance of 2012 Series X preferred stock, net of issuance costs			2,279
Non-cash financing activities:
Reclassification of 2012 Series X preferred stock from debt to preferred stock			$ 2,486

Nature of Organization and Operations	12 Months Ended
Nature of Organization and Operations	Dec. 31, 2015
Accounting Policies [Abstract]
Nature of Organization and Operations	1. Nature of Organization and Operations Akebia Therapeutics, Inc. (Akebia, or the Company) is a biopharmaceutical focused on the development of novel proprietary therapeutics based on hypoxia inducible factor, or HIF, biology and the commercialization of these products for patients with serious unmet medical needs. HIF is the primary regulator of the production of red blood cells, or RBCs, in the body and a potentially novel mechanism for the treatment of anemia secondary to chronic kidney disease, or CKD. Pharmacologic modulation of the HIF pathway may also have broader therapeutic applications in acute renal failure, organ protection, ischemia-reperfusion injury, cancer, ophthalmology, and inflammatory diseases. The Company’s lead product candidate, vadadustat, formerly known as AKB-6548, is a promising cost-effective alternative for the treatment of anemia. Vadadustat is being developed as a once-daily, oral therapy and has successfully completed Phase 2 development demonstrating that vadadustat can safely and predictably raise hemoglobin levels in patients with anemia related to CKD. The Company’s operations to date have been limited to organizing and staffing the Company, business planning, raising capital, acquiring and developing its technology, identifying potential product candidates and undertaking preclinical and clinical studies. The Company has not generated any product revenue to date, nor is there any assurance of any future product revenue. The Company’s product candidates are subject to long development cycles and there is no assurance the Company will be able to successfully develop, obtain regulatory approval for or market its product candidates. The Company is subject to a number of risks including, but not limited to, the need to obtain adequate additional funding including the resources necessary to fund its recently commenced global Phase 3 development of vadadustat, in dialysis and non-dialysis patients. In December 2015, the Company began dosing patients in its Phase 3 vadadustat program in non-dialysis patients with anemia related to CKD, PRO₂TECT, after obtaining feedback from United States and European regulatory authorities regarding the design of the program. The Company also expects to initiate its Phase 3 vadadustat program in dialysis-dependent CKD patients, INNO₂VATE, in 2016, anticipating full enrollment by early 2018. The Company has engaged a clinical research organization for the PRO₂TECT and INNO₂VATE programs. The Company expects the cost of the Phase 3 program to be in the range of $80,000 to $85,000 per patient and it plans to enroll approximately 3,100 patients in PRO₂TECT and approximately 2,600 patients in INNO₂VATE. The Company is also subject to a number of risks including the need to obtain adequate additional funding, possible failure of preclinical testing or clinical trials, the need to obtain marketing approval for its product candidates, the development of new technological innovations by competitors, the need to successfully commercialize and gain market acceptance of any of the Company’s products that are approved and the ability to protect its proprietary technology. If the Company does not successfully commercialize any of its products, it will be unable to generate product revenue or achieve profitability. On March 25, 2014, the Company completed its initial public offering, or IPO, whereby the Company sold 6,762,000 shares of common stock, including 879,647 shares of common stock pursuant to the full exercise of an over-allotment option granted to the underwriters in connection with the offering, at a price of $17.00 per share. The shares began trading on the NASDAQ Global Market on March 20, 2014. The aggregate net proceeds received by the Company from the offering were $104.4 million, net of underwriting discounts and commissions and estimated offering expenses. Upon the closing of the IPO, all outstanding shares of convertible redeemable preferred stock converted into 12,115,183 shares of common stock. Additionally, the Company is now authorized to issue up to 175,000,000 shares of common stock and 25,000,000 shares of undesignated preferred stock. In April 2015, the Company completed a follow-on public offering whereby the Company sold 8,363,636 shares of common stock, including 1,090,909 share of common stock pursuant to the full exercise of an over-allotment granted to the underwriters in connection with the offering, at a price of $8.25 per share. The aggregate net proceeds received by the Company from the offering were approximately $64.6 million, net of underwriting discounts and commissions and estimated offering expenses. In August 2015, the Company entered into a Sales Agreement with Cantor Fitzgerald & Co. to periodically sell up to $50 million of shares of the Company’s common stock in an “at-the-market” (ATM) offering. In January 2016, the Company terminated the Sales Agreement in order to use the shares for the January 2016 follow-on offering. During 2015, prior to the termination, the Company sold 1,719,434 shares of common stock pursuant to the Sales Agreement. The aggregate net proceeds received by the Company were approximately $18.4 million, net of commissions. No additional shares of the Company’s common stock will be sold pursuant to the Sales Agreement. In December 2015, we entered into a collaboration agreement with Mitsubishi Tanabe to develop and commercialize vadadustat in Japan and certain other countries in Asia for total milestone payments of up to $350 million, including up to $100 million in upfront and development payments, of which $40.0 million was received in January 2016. Of the $40.0 million received, $20.0 million is subject to refund to Mitsubishi depending on the outcome of discussions with the Japanese regulator about the global trial design. In addition, we will receive tiered double-digit royalty payments on sales of vadadustat. In January 2016, the Company completed a follow-on public whereby the Company sold 7,250,000 shares of common stock at a price of $9.00 per share. The aggregate net proceeds received by the Company from the offering were approximately $61.0 million, net of underwriting discounts and commissions and estimated offering expenses payable by the Company. The Company believes that it can continue as a going concern as its cash resources of approximately $138.5 million at December 31, 2015 will be sufficient to allow the Company to fund its current operating plan through at least the next twelve months. There can be no assurance, however, that the current operating plan will be achieved in the timeframe anticipated by the Company, or that its cash resources will fund the Company’s operating plan for the period anticipated by the Company or that additional funding will be available on terms acceptable to the Company, or at all. Unless otherwise indicated, all information in these condensed consolidated financial statements gives retrospective effect to the 1.75-for-1 stock split of the Company’s common stock (the Stock Split) that was effected on March 6, 2014 (see Note 6), as well as any other stock-splits in historical periods. The Company was incorporated on February 27, 2007 under the laws of the State of Delaware.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2015

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, Akebia Therapeutics Securities Corporation and Akebia Europe Limited. All intercompany balances and transactions have been eliminated in consolidation. These consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”).

In the quarter ended December 31, 2015, the Company identified and corrected an error in the historical classification of certain operating costs between research and development and general and administrative expenses. The Company concluded the effect of this classification error was not material to its consolidated financial statements for any prior period. The classification correction had no effect on the Company’s current or historical total operating expenses or net loss.

Recent Accounting Pronouncements

From time to time, new accounting pronouncements are issued by the FASB or other standard-setting bodies and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes the impact of recently issued standards that are not yet effective will not have a material impact on its financial position or results of operations upon adoption.

In November 2015, the FASB issued ASU No. 2015-17, to simplify the presentation of deferred income taxes. The new standard requires that all deferred tax assets and liabilities, along with any related valuation allowance, be classified as noncurrent on the balance sheet. The standard is effective for interim and annual periods beginning after December 15, 2016 and allows for early adoption using a full retrospective method or a prospective method for all periods presented. The Company has elected to early adopt the provisions of this new standard in the fourth quarter of 2015 using a full retrospective method. The accounting standard did not have any impact on the Company’s consolidated financial statements since a full valuation allowance has been provided on the Company’s deferred tax assets (see Note 7).

In August 2014, the FASB issued ASU 2014-15, which requires management of public and private companies to evaluate whether there is substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued. Management’s evaluation should be based on relevant conditions and events that are known and reasonably knowable at the date that the financial statements are issued. If conditions or events raise substantial doubt about an entity’s ability to continue as a going concern, and substantial doubt is not alleviated after consideration of management’s plans, an entity should include a statement in the footnotes indicating that there is substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued. The new standard is effective for annual periods ending after December 15, 2016, and for annual periods and interim periods thereafter. Early application is permitted. The Company has concluded, that if this standard had been adopted as of December 31, 2015, substantial doubt about the Company’s ability to continue as a going concern does not exist.

In May 2014, the FASB, issued a new revenue recognition standard which amends revenue recognition principles and provides a single, comprehensive set of criteria for revenue recognition within and across all industries. The new standard provides a five step framework whereby revenue is recognized when promised goods or services are transferred to a customer at an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The standard also requires enhanced disclosures pertaining to revenue recognition in both interim and annual periods. In August 2015, the FASB deferred the effective date of the new revenue standard from January 1, 2017 to January 1, 2018. Early adoption is permitted any time after the original effective date, which for us is January 1, 2017. The standard allows for adoption using a full retrospective method or a modified retrospective method. We are currently evaluating the timing, method of adoption and the expected impact that the standard could have on our consolidated financial statements and related disclosures.

Segment Information

Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company views its operations and manages its business in one operating segment, which is the business of developing and commercializing proprietary therapeutics based on HIF biology.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes, and management must select an amount that falls within that range of reasonable estimates. Estimates are used in the following areas, among others: prepaid and accrued research and development expense, stock-based compensation expense, accrued expenses and income taxes.

Prior to the IPO, the Company utilized significant estimates and assumptions in determining the fair value of its common stock. The Company granted stock options at exercise prices not less than the fair market value of its common stock as determined by the Board of Directors contemporaneously at the date such grants were made, with input from management. Prior to the Company’s IPO in March 2014, the fair value of common stock at the grant date was adjusted in connection with the Company’s retrospective fair value assessment for financial reporting purposes. Accordingly, the Board of Directors determined the estimated fair value of the Company’s common stock based on a number of objective and subjective factors, including external market conditions affecting the biotechnology industry sector and the prices at which the Company sold shares of preferred stock, the superior rights and preferences of securities senior to the Company’s common stock at the time and the likelihood of achieving a liquidity event, such as an IPO or sale of the Company.

The Company utilized various valuation methodologies in accordance with the framework of the American Institute of Certified Public Accountants Technical Practice Aid, Valuation of Privately-Held Company Equity Securities Issued as Compensation, to estimate the fair value of its common stock in periods prior to March 2014. The methodologies included a probability analysis including both a potential public trading scenario and potential sale scenario. In both scenarios, value is estimated using the guideline public company method. The sale scenario includes an adjustment for a market participant acquisition premium. Value is allocated among the preferred and common shares according to the rights associated with each type of security. Valuation methodologies include estimates and assumptions that require the Company’s judgment. These estimates include assumptions regarding future performance, including the successful completion of a public offering. Significant changes to the key assumptions used in the valuations could result in different fair values of common stock at each valuation date.

Cash and Cash Equivalents

Cash and cash equivalents consist of all cash on hand, deposits and funds invested in available-for-sale securities with original maturities of three months or less at the time of purchase. At December 31, 2015, the Company’s cash is primarily in money market funds. The Company may maintain balances with its banks in excess of federally insured limits.

Investments

Management determines the appropriate classification of securities at the time of purchase and reevaluates such designation as of each balance sheet date. Currently, the Company classifies all securities as available-for-sale which are included in current assets as they are intended to fund current operations. The Company carries available-for-sale securities at fair value. The Company conducts periodic reviews to identify and evaluate each investment that has an unrealized loss, in accordance with the meaning of other-than-temporary impairment and its application to certain investments. When assessing whether a decline in the fair value of a security is other-than-temporary, the Company considers the fair market value of the security, the duration of the security’s decline, and prospects for the underlying business. Based on these considerations, the Company did not identify any other-than-temporary unrealized losses at December 31, 2015. Unrealized losses on available-for-sale securities that are determined to be temporary, and not related to credit loss, are recorded in accumulated other comprehensive loss, a component of stockholders’ equity. The amortized cost of debt securities in this category reflects amortization of premiums and accretion of discounts to maturity computed under the effective interest method. The Company includes this amortization in the caption “Interest income, net” within the Consolidated Statements of Operations and Comprehensive Loss. We also include in net investment income, realized gains and losses and declines in value determined to be other than temporary. The Company bases the cost of securities sold upon the specific identification method, and includes interest and dividends on securities in interest income.

Patents

Costs incurred in connection with the application for and issuance of patents are expensed as incurred.

Income Taxes

Income taxes are recorded in accordance with FASB Topic 740, Income Taxes (ASC 740), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided, if, based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company has elected to early adopt the provisions of ASU No. 2015-17 in the fourth quarter of 2015 using a full retrospective method. As a result, all deferred taxes as of December 31, 2015 and 2014 are classified as noncurrent within the income tax provision (see Note 7), however as we record a full valuation allowance against the Company’s net deferred tax assets, the adoption of this standard has no impact on our consolidated balance sheets as of December 31, 2015 and 2014.

The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position, as well as consideration of the available facts and circumstances. As of December 31, 2015 and 2014, the Company does not have any significant uncertain tax positions. The Company recognizes interest and penalties related to uncertain tax positions in income tax expense.

Stock-Based Compensation

The Company accounts for its stock-based compensation awards in accordance with FASB ASC Topic 718, Compensation—Stock Compensation (ASC 718). ASC 718 requires all stock-based payments to employees, including grants of employee stock options, restricted stock, restricted stock units, or RSUs, and modifications to existing stock awards, to be recognized in the statements of operations and comprehensive loss based on their fair values. The Company accounts for stock-based awards to non-employees in accordance with FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees (ASC 505-50), which requires the fair value of the award to be re-measured at fair value until a performance commitment is reached or counterparty performance is complete. The Company’s stock-based awards are comprised of stock options, shares of restricted stock and shares of common stock. The Company estimates the fair value of options granted using the Black-Scholes option pricing model. The Company uses the quoted market price of comparable public companies to determine the fair value of restricted stock awards and common stock awards.

The Black-Scholes option pricing model requires the input of certain subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to the lack of a public market for the trading of the Company’s common stock and a lack of company-specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The computation of expected volatility is based on the historical volatility of a representative group of companies with similar characteristics to the Company, including stage of product development and life science industry focus. The Company is in the product development stage with no revenue and the representative group of companies has certain similar characteristics to the Company. The Company believes the group selected has sufficient similar economic and industry characteristics, and includes companies that are most representative of the Company. The Company uses the simplified method as prescribed by the SEC Staff Accounting Bulletin No. 107, Share-Based Payment, to calculate the expected term for options granted to employees as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. The expected term is applied to the stock option grant group as a whole, as the Company does not expect substantially different exercise or post-vesting termination behavior among its employee population. For options granted to non-employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected life of the stock options. The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock, which is similar to the Company’s peer group.

The Company’s stock-based awards are subject to either service- or performance-based vesting conditions. Compensation expense related to awards to employees with service-based vesting conditions is recognized on a straight-line basis based on the grant date fair value over the associated service period of the award, which is generally the vesting term. Consistent with the guidance in ASC 505- 50, compensation expense related to awards to non-employees with service-based vesting conditions is recognized on a straight-line basis based on the then-current fair value at each financial reporting date prior to the measurement date over the associated service period of the award, which is generally the vesting term. Compensation expense related to awards to employees with performance-based vesting conditions is recognized based on the grant date fair value over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable. Consistent with the guidance in ASC 505-50, compensation expense related to awards to non-employees with performance-based vesting conditions is recognized based on the then-current fair value at each financial reporting date prior to the measurement date over the requisite service period using the accelerated attribution method to the extent achievement of the performance condition is probable.

The Company is also required to estimate forfeitures at the time of grant, and revise those estimates in the subsequent periods if actual forfeitures differ from its estimates. The Company uses historical data to estimate pre-vesting forfeitures and record stock-based compensation expense only for those awards that are expected to vest. To the extent that actual forfeitures differ from the Company’s estimates, the difference is recorded as a cumulative adjustment in the period the estimates were revised. Stock-based compensation expense recognized in the condensed consolidated financial statements is based on awards that are ultimately expected to vest.

Fair Value of Financial Instruments

The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. FASB ASC Topic 820, Fair Value Measurements and Disclosures (ASC 820), establishes a hierarchy of inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the observable inputs be used when available.

Observable inputs are inputs that market participants would use in pricing the asset or liability based on market data obtained from sources independent of the Company. Unobservable inputs are inputs that reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability, and are developed based on the best information available in the circumstances. The fair value hierarchy applies only to the valuation inputs used in determining the reported fair value of the investments, and is not a measure of the investment credit quality. The three levels of the fair value hierarchy are described below:

Level 1 – Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

Level 2 – Valuations based on quoted prices for similar assets or liabilities in markets that are not active, or for which all significant inputs are observable, either directly or indirectly.

Level 3 – Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable.

To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

Items measured at fair value on a recurring basis include short-term investments (see Note 4). The carrying amounts of accounts receivable, prepaid expenses and other current assets, accounts payable and accrued expenses approximate their fair values due to their short-term maturities. The rate implicit within the Company’s capital lease obligation approximates market interest rates.

Concentrations of Credit Risk and Off-Balance Sheet Risk

Cash, investments and accounts receivable are the only financial instruments that potentially subject the Company to concentrations of credit risk. The Company maintains its cash with high quality, accredited financial institutions and, accordingly, such funds are subject to minimal credit risk. The Company has no significant off-balance sheet concentrations of credit risk, such as foreign currency exchange contracts, option contracts or other hedging arrangements.

Net Loss per Share

Basic net loss per share is calculated by dividing net loss attributable to common stockholders by the weighted-average shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by adjusting weighted-average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. For purposes of the diluted net loss per share calculation, preferred stock, stock options, unvested restricted stock and RSUs are considered to be common stock equivalents, but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented.

Property and Equipment

Property and equipment is stated at cost, less accumulated depreciation. Assets under capital lease are included in property and equipment. Property and equipment is depreciated using the straight-line method over the estimated useful lives of the assets, generally three to seven years. Such costs are periodically reviewed for recoverability when impairment indicators are present. Such indicators include, among other factors, operating losses, unused capacity, market value declines and technological obsolescence. Recorded values of asset groups of equipment that are not expected to be recovered through undiscounted future net cash flows are written down to current fair value, which generally is determined from estimated discounted future net cash flows (assets held for use) or net realizable value (assets held for sale).

The following is the summary of property and equipment and related accumulated depreciation as of December 31, 2015 and 2014.

	Useful Life	2015			2014
		(in thousands)
Computer equipment and software	3	$	300		$	99
Furniture and fixtures	5		243			117
Equipment	7		50			6
Leasehold improvements	Shorter of the useful life or remaining lease term (3 years)		70			27
Office equipment under capital lease	3		24			12
			687			261
Less accumulated depreciation			(147	)		(51	)
Net property and equipment		$	540		$	210

Depreciation expense, including expense associated with assets under capital leases, was approximately $96,000, $49,000 and $1,000 for the years ended December 31, 2015, 2014 and 2013, respectively.

Available for Sale Securities

12 Months Ended

Dec. 31, 2015

Investments Debt And Equity Securities [Abstract]

Available for Sale Securities

3. Available for sale securities

Available for sale securities at December 31, 2015 and 2014 consist of the following:

			Gross		Gross
			Unrealized		Unrealized
	Amortized Cost		Gains		Losses			Fair Value
	(in thousands)
December 31, 2015
Cash and cash equivalents	$	49,778	$	—	$	—		$	49,778
Available for sale securities:
Certificates of deposit	$	21,505	—		—			$	21,505
U.S. Government debt securities		46,461	—			(185	)		46,276
Corporate debt securities		20,944		1		(50	)		20,895
Total available for sale securities	$	88,910	$	1	$	(235	)	$	88,676
Total cash, cash equivalents, and available for sale securities	$	138,688	$	1	$	(235	)	$	138,454

The estimated fair value of the Company’s available-for-sale securities balance at December 31, 2015, by contractual maturity, is as follows:

Due in one year or less	$	39,075
Due after one year		49,601
Total available for sale securities	$	88,676

			Gross		Gross
			Unrealized		Unrealized
	Amortized Cost		Gains		Losses			Fair Value
	(in thousands)
December 31, 2014
Cash and cash equivalents	$	32,780	$	—	$	—		$	32,780
Available for sale securities:
Certificates of deposit	$	13,429	—			—		$	13,429
U.S. Government debt securities		38,412		1		(28	)		38,385
Commercial paper		2,499		—		—			2,499
Corporate debt securities		21,854		3		(32	)		21,825
Total available for sale securities	$	76,194	$	4	$	(60	)	$	76,138
Total cash, cash equivalents, and available for sale securities	$	108,974	$	4	$	(60	)	$	108,918

- Definition

The entire disclosure for investments in certain debt and equity securities.

+ References + Details

Fair Value of Financial Instruments

12 Months Ended

Dec. 31, 2015

Fair Value Disclosures [Abstract]

Fair Value of Financial Instruments

4. Fair Value of Financial Instruments

The Company utilizes a portfolio management company for the valuation of the majority of its investments. This company is an independent, third-party vendor recognized to be an industry leader with access to market information that obtains or computes fair market values from quoted market prices, pricing for similar securities, recently executed transactions, cash flow models with yield curves and other pricing models. For valuations obtained from the pricing service, the Company performs due diligence to understand how the valuation was calculated or derived, focusing on the valuation technique used and the nature of the inputs.

Based on the fair value hierarchy, the Company classifies its cash equivalents and available-for-sale securities within Level 1 or Level 2. This is because the Company values its cash equivalents and available-for-sale securities using quoted market prices or alternative pricing sources and models utilizing market observable inputs.

Assets measured or disclosed at fair value on a recurring basis as of December 31, 2015 are summarized below:

	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets:
Cash and cash equivalents	$	49,778	$	—	$	—	$	49,778
Certificates of deposit		—		21,505		—		21,505
U.S. Government debt securities		—		46,276		—		46,276
Corporate debt securities		—		20,895		—		20,895
	$	49,778	$	88,676	$	—	$	138,454

The Company’s corporate debt securities are all investment grade.

Assets measured or disclosed at fair value on a recurring basis as of December 31, 2014 are summarized below:

	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets:
Cash and cash equivalents	$	32,780	$	—	$	—	$	32,780
Certificates of deposit		—		13,429		—		13,429
U.S. Government debt securities		—		38,385		—		38,385
Commercial paper		—		2,499		—		2,499
Corporate debt securities		—		21,825		—		21,825
	$	32,780	$	76,138	$	—	$	108,918

The Company had no assets or liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) at December 31, 2015 and December 31, 2014.

Investment securities are exposed to various risks such as interest rate, market and credit. Due to the level of risk associated with certain investment securities and the level of uncertainty related to changes in the value of investment securities, it is at least reasonably possible that changes in risks in the near term would result in material changes in the fair value of investments.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Accrued Expenses

12 Months Ended

Dec. 31, 2015

Payables And Accruals [Abstract]

Accrued Expenses

5. Accrued Expenses

Accrued expenses are as follows:

	December 31,
	2015		2014
	(in thousands)
Accrued clinical	$	4,536	$	2,132
Accrued bonus		2,178		1,286
Professional fees		647		328
Accrued payroll		518		213
Accrued vacation		310		177
Accrued severance	—			179
Other		1,366		549
Total accrued expenses	$	9,555	$	4,864

In February 2014, the Company entered into a separation agreement with an employee primarily as a result of the transition to the Company’s Cambridge, Massachusetts location. During the first quarter of 2014, the Company recorded severance expense in the amount of approximately $0.3 million, which was recorded to general and administrative expense. During the year ended December 31, 2014, approximately $0.3 million was paid out of the severance accrual and the remainder was paid during the first quarter of 2015. At December 31, 2015 and 2014, approximately $0 and $45,890, respectively, remained in accrued expenses in relation to the unpaid severance costs.

In August 2014, the Company entered into a separation agreement with an employee, which became effective on August 13, 2014. The Company will record the expense and liability associated with the separation agreement ratably over the period from August 5, 2014 through December 31, 2015 because the severance payments are subject to continued service and forfeiture until December 31, 2015. During the year ended 2015 and 2014, the Company recorded severance expense in the amount of approximately $0.3 million and $0.1 million respectively, which was recorded to research and development expense. At December 31, 2015 and 2014, approximately $0 and $0.1 million, respectively, remained in accrued expense in relation to the unpaid severance costs.

Stockholders' Equity

12 Months Ended

Dec. 31, 2015

Equity [Abstract]

Stockholders' Equity

6. Stockholders’ Equity

Authorized and Outstanding Capital Stock

As of December 31, 2014, the authorized capital stock of the Company included 175,000,000 shares of common stock, par value $0.00001 per share, of which 30,662,218 and 20,370,624 shares are issued and outstanding at December 31, 2015 and December 31, 2014, respectively; and 25,000,000 shares of undesignated preferred stock, par value $0.00001 per share, of which 0 shares are issued and outstanding at December 31, 2015 and December 31, 2014.

On March 6, 2014, the Company effected a 1.75-for-1 stock split of its outstanding common stock. Unless otherwise indicated, all share data and per share amounts in these financial statements have been retroactively adjusted to reflect the stock split, as well as any stock splits that occurred in periods prior to March 6, 2014.

Upon the closing of the IPO on March 25, 2014, all of the outstanding shares of the Company’s redeemable convertible preferred stock were converted into 12,115,183 shares of its common stock. As of December 31, 2014, the Company does not have any redeemable convertible preferred stock issued or outstanding.

Equity Plans

On February 28, 2014, the Company’s Board of Directors adopted its 2014 Incentive Plan (2014 Plan) and its 2014 Employee Stock Purchase Plan (ESPP), which were subsequently approved by its stockholders and became effective upon the closing of the Company’s IPO on March 25, 2014. The 2014 Plan replaces the 2008 Equity Incentive Plan (2008 Plan), however, any options or awards outstanding under the 2008 Plan at the time of adoption of the 2014 Plan remain outstanding and effective.

The 2014 Plan allows for the granting of stock options, stock appreciation rights, or SARs, restricted stock, unrestricted stock, stock units, performance awards and other awards convertible into or otherwise based on shares of our common stock. Dividend equivalents may also be provided in connection with an award under the 2014 Plan. The Company’s employees, officers, directors and consultants and advisors are eligible to receive awards under the 2014 Plan. The Company initially reserved 1,785,000 shares of its common stock for the issuance of awards under the 2014 Plan. The 2014 Plan provides that the number of shares reserved and available for issuance under the 2014 Plan will automatically increase annually on January 1^st of each calendar year, by an amount equal to three percent (3%) of the number of shares of stock outstanding on a fully diluted basis as of the close of business on the immediately preceding December 31^st(2014 Plan Evergreen Provision). The Company’s Board of Directors may act prior to January 1^st of any year to provide that there will be no automatic increase in the number of shares available for grant under the 2014 Plan for that year (or that the increase will be less than the amount that would otherwise have automatically been made). Subject to adjustment, no more than 1,131,937 shares of our common stock may be delivered in satisfaction of incentive stock options awarded under the 2014 Plan. During the year ended December 31, 2015, the Company granted 957,050 stock options to employees, 27,875 restricted stock units to employees and 35,000 stock options to directors under the 2014 Plan.

The ESPP authorizes the initial issuance of up to a total of 262,500 share of the Company’s common stock to participating employees. The maximum aggregate number of shares of common stock available for purchase pursuant to the exercise of options granted under the ESPP will be the lesser of (a) 262,500 shares, increased on each anniversary of the adoption of the ESPP by one percent (1%) of the total shares of common stock then outstanding and (b) 739,611 shares (which is equal to five percent (5%) of the total shares of common stock outstanding on the date of the adoption of the ESPP on a fully diluted, as converted basis (ESPP Evergreen Provision). Under the ESPP, each offering period is six months, at the end of which employees may purchase shares of common stock through payroll deductions made over the term of the offering. The per-share purchase price at the end of each offering period is equal to the lesser of 85% of the closing price of our common stock at the beginning or end of the offering period.

Shares Reserved for Future Issuance

The Company has reserved for future issuance the following number of shares of common stock:

	December 31,
	2015		2014
Unvested restricted common stock		216,716		422,145
Common stock options and RSU's outstanding		2,231,057		1,526,346
Shares available for issuance under the 2014 Plan ⁽¹⁾		1,318,732		1,549,154
Shares available for issuance under the ESPP		440,304		262,500
Total		4,206,809		3,760,145

⁽¹⁾

On January 1, 2016, the shares reserved for future grants under the 2014 Plan increased by 986,800 shares pursuant to the 2014 Plan Evergreen Provision.

⁽²⁾

On February 28, 2016, the shares reserved for future issuance under the ESPP increased by 379,430 shares pursuant to the ESPP Evergreen Provision.

Stock-Based Compensation

Stock Options

On March 6, 2015, the Company issued 479,750 stock options to employees. Options granted by the Company vest over periods of between 12 and 48 months. Options vest in installments of (i) 25% at the one year anniversary and (ii) in either 36 or 48 equal monthly or 12 equal quarterly installments beginning in the thirteenth month after the initial Vesting Commencement Date (as defined) or grant date, subject to the employee’s continuous service with the Company. Options generally expire ten years after the date of grant.

The assumptions used in the Black-Scholes pricing model to estimate the grant date fair value of options granted to employees are as follows:

	Year ended December 31,
	2015		2014
Risk-free interest rate	1.44% - 1.95%		1.63% - 2.06%
Dividend yield		0.00%		0.00%
Volatility	62.47% - 81.25%		67.97% - 77.00%
Expected term (years)	5.51 - 6.25		6.25

The following table summarizes the Company’s stock option activity:

	Shares			Weighted-Average Exercise Price		Weighted-Average Contractual Life (in years)		Aggregate Intrinsic Value
Outstanding, December 31, 2014		1,526,349		$	7.57			$	11,523,531
Granted		992,050		$	9.81
Exercised		(218,674	)	$	0.60			$	2,047,507
Forfeited		(93,090	)	$	12.96			$	107,384
Expired/cancelled		—
Outstanding, December 31, 2015		2,206,635		$	9.04		8.20	$	13,114,811
Options exercisable, December 31, 2015		610,696		$	6.20		6.63	$	5,620,564
Expected to vest, December 31, 2015		1,958,808		$	9.71		8.56	$	10,737,600

As of December 31, 2015, there was approximately $9.7 million of unrecognized compensation cost related to stock options, which is expected to be recognized over a weighted average period of 2.62 years.

Restricted Stock

On December 23, 2013, the Company issued 450,224 shares of restricted stock to employees and 79,067 shares of restricted stock to non-employees at a grant date fair value of $7.42 per share. The aggregate grant date fair value for the shares of restricted stock issued on December 23, 2013 totaled approximately $3.9 million. The awards of restricted stock contained a performance condition wherein vesting is contingent upon the Company’s consummation of a liquidity event, as defined, prior to the fifth anniversary of the date of grant. Certain of the awards of restricted stock have a requisite service period that was complete upon grant. The remainder of the awards of restricted stock have a requisite service period of four years whereby the award vests 25% on the one year anniversary of the Vesting Commencement Date (as defined), then ratably on the first day of each calendar quarter for 12 quarters, subject to continuous service by the individual and achievement of the performance target. Due to the nature of the performance condition, the Company had concluded that the performance condition was not probable of achievement and therefore, recognition of compensation cost had been deferred until the occurrence of a liquidity event, as defined. The liquidity event occurred upon the closing of the Company’s IPO on March 25, 2014. Accordingly, the Company recognized $0.1 million of compensation expense on March 25, 2014 related to the restricted stock awards with a requisite service period that was complete upon grant. Compensation expense related to the remainder of the restricted stock awards is being recognized over the associated requisite service period commencing on March 25, 2014. The Company recorded approximately $0.9 million of stock-based compensation expense related to restricted stock during 2015.

A summary of the Company’s restricted stock activity is as follows:

	Shares			Weighted-Average Grant Date Fair Value
Unvested balance, December 31, 2014		422,145		$	8.39
Granted		-
Vested		(169,376	)	$	8.87
Forfeited		(36,053	)	$	8.93
Outstanding, December 31, 2015		216,716		$	7.97

As of December 31, 2015, there was approximately $0.4 million of unrecognized compensation cost related to the restricted stock awards granted on December 23, 2013 with a performance condition. The recognition of the compensation cost for these awards did not begin until the closing of the IPO on March 25, 2014. The unrecognized compensation cost is expected to be recognized over a weighted average period of 1.08 years.

Common Stock Awards

In connection with the termination of a former employee in September 2013, the Company granted the former employee stock awards totaling 70,964 shares in September 2013 and 105,753 shares in December 2013 of Common Stock, at a fair value of $3.77 per share and $7.42 per share, respectively. The fair value of the awards are based on the estimated fair value of the Company’s Common Stock at the date of grant. Accordingly, compensation cost was recognized in full on the date of grant. The associated common shares immediately vested, and were not subject to any other restriction. Accordingly, the compensation cost was recognized in full on the date of grant. Total compensation cost of approximately $1.1 million was recognized during the year ended December 31, 2013 related to these awards.

Restricted Stock Units

On March 6, 2015, the Company issued 27,875 RSUs to employees. The RSUs vest 100% on the three year anniversary. Total stock-compensation expense to be recognized over the life of the RSUs is $0.3 million and will be recognized on a straight-line basis over the vesting period. The Company recorded approximately $62,000 of stock-based compensation expense related to the RSUs during 2015.

	Shares			Weighted-Average Grant Date Fair Value
Unvested balance, December 31, 2014		-		$	-
Granted		27,875		$	11.15
Vested		-		$	-
Forfeited		(3,450	)	$	11.15
Outstanding, December 31, 2015		24,425		$	11.15

As of December 31, 2015, there was approximately $0.2 million of unrecognized compensation cost related to restricted stock units, which is expected to be recognized over a weighted average period of 2.18 years.

ESPP

The first offering period under the ESPP opened on January 2, 2015. The Company issued 25,903 shares during the year ended December 31, 2015. The Company recorded approximately $83,000 of stock-based compensation expense related to ESPP during 2015.

Compensation Expense Summary

The Company has recognized the following compensation cost related to share-based awards:


	Year ended December 31,
	2015		2014
	(in thousands)
Research and development	$	2,079	$	2,766
General and administrative		2,635		3,244
Total	$	4,714	$	6,010

Compensation expense by type of award:


	Year ended December 31,
	2015		2014
	(in thousands)
Stock options	$	3,660	$	1,925
Restricted stock		909		4,085
Restricted stock units		62		—
Employee stock purchase plan		83		—
Total	$	4,714	$	6,010

Included in the compensation expense for the year ended December 31, 2014, is approximately $1.0 million related to the modification of awards in connection with an employee separation agreement in the first quarter of 2014.

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(d),(e))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB TOPIC 4.E)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74512-122707

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21488-112644

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29-31)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB TOPIC 4.C)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187143-122770

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6405834&loc=d3e23285-112656

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Preferred Stock
-URI http://asc.fasb.org/extlink&oid=6521494

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section E

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section C

+ Details

Income Taxes

12 Months Ended

Dec. 31, 2015

Income Tax Disclosure [Abstract]

Income Taxes

7. Income Taxes

The Company’s income tax provision was computed based on the federal statutory rate and the average state statutory rates, net of the related federal benefit. There was no current or deferred income tax expense or benefit for the years ended December 31, 2015 and 2014, due to the Company’s net losses and increases in its deferred tax asset valuation allowance. Following is a reconciliation of the statutory federal income rate to the Company’s effective tax rate:

	Year ended December 31,
	2015			2014
Federal tax at statutory rate		34.0	%		34.0	%
State and local tax at statutory rate		3.0			5.5
Research and development tax credits		0.4			(0.6	)
Equity compensation		(0.6	)		(1.6	)
Other permanent differences and credits		—			(0.1	)
Change in valuation allowance		(36.8	)		(37.2	)
Effective tax rate		0.0	%		0.0	%

The Company’s income tax provision was computed based on the federal statutory rate and the average state statutory rates, net of the related federal benefit.

Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. When realization of the deferred tax asset is more likely than not to occur, the benefit related to the deductible temporary differences attributable to operations is recognized as a reduction of income tax expense. Valuation allowances are provided against deferred tax assets when, based on all available evidence, it is considered more likely than not that some portion or all of the recorded deferred tax assets will not be realized in future periods. The Company cannot be certain that future taxable income will be sufficient to realize its deferred tax assets, and accordingly a full valuation allowance has been provided on its deferred tax assets. The Company continues to maintain the underlying tax benefits to offset future taxable income and to monitor the need for a valuation allowance based on the profitability of its future operations. The valuation allowance increased by approximately $22.3 million and $13.8 million, during the years ended December 31, 2015 and 2014, respectively, primarily as a result of increases in unbenefited deferred tax assets such as tax losses and stock-based compensation. Significant components of the Company’s deferred tax assets and liabilities are as follows:

		December 31,
		2015			2014
	(in thousands)
Deferred tax assets:
Accrued expenses		$	912		$	559
Intangible assets			527		$	603
Restricted stock			301		$	436
Fixed assets			—		$	2
Non-qualified stock options ⁽¹⁾			1,558		$	440
Research and development credits			1,829		$	1,589
Net operating loss carryforward			51,356		$	30,559
Other			64		$	39
Total deferred tax assets			56,547		$	34,227
Less valuation allowance			(56,545	)		(34,227	)
Total deferred tax assets, net of valuation allowance			2			—
Deferred tax liabilities:
Fixed assets			(2	)		—
Restricted stock			—			—
Total deferred tax liabilities			(2	)		—
Net deferred tax asset		$	—		$	—

⁽¹⁾

This amount was reclassified from “other” in 2014 for comparability.

At December 31, 2015 and December 31, 2014, the Company has approximately $1.0 million (after amortization of $0.9 million) and $1.2 million (after amortization of $0.8 million), respectively, of start-up expenses capitalized for income tax purposes with amortization available to offset future federal, state and local income tax. At December 31, 2015 and 2014, the Company has approximately $140.5 million and $84.2 million, respectively, of federal net operating loss (NOL) carry-forwards. Additionally, at December 31, 2015 and 2014, the Company has approximately $111.9 million and $83.7 million, respectively, of state net operating loss (NOL) carry-forwards which expire through 2035. Included in the 2015 and 2014 NOLs are approximately $0.9 million of tax deductions related to equity compensation in excess of book compensation expense. Pursuant to the realization requirements in ASC 718, these tax deductions are not included in the NOL deferred tax asset above. The Company also has approximately $2.0 million of federal and state research and development tax credit carry-forwards. The NOL and research and development tax credit carry-forwards begin to expire in 2027 through 2035, and will be utilized for tax purposes at such time the Company generates taxable income. The NOL and research and development tax credit carry-forwards may be limited in certain circumstances, including ownership changes.

Under the provisions of the Internal Revenue Code, the net operating loss and tax credit carry-forwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating loss and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has completed several financings since its inception, which may have resulted in a change in control as defined by Sections 382 and 383 of the Internal Revenue Code, or could result in a change in control in the future. The Company has not yet analyzed whether there has been a change in control.

For applicable years, the Company generated research credits but has not conducted a study to document its qualified activities. This study may result in an adjustment to the Company’s research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company’s research and development credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the deferred tax asset established for the research and development credit carry-forwards and the valuation allowance.

The Company will recognize interest and penalties related to uncertain tax positions in income tax expense. As of December 31, 2015, 2014 and 2013, the Company had no accrued uncertain tax positions or associated interest or penalties and no amounts have been recognized in the Company’s consolidated statements of operations.

The Company files income tax returns in the U.S. federal jurisdiction, and various state jurisdictions. The Company’s 2012 through 2014 tax years remain open and subject to examination by federal and state taxing authorities. However, federal and state net operating losses from 2008 through 2014 are subject to review by taxing authorities in the year utilized.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2015

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

8. Commitments and Contingencies

In December 2013, the Company entered into a three-year lease for 6,837 square feet of office space in Cambridge, Massachusetts. The lease has monthly lease payments of approximately $31,000 for the first twelve months, with annual rent escalation thereafter, and provides a rent abatement of approximately $31,000 for the first full calendar month of the lease term. The lease term commenced and rental payments began in January 2014. The Company recorded a deferred lease obligation in 2014 which represents the cumulative difference between actual facility lease payments and lease expense recognized ratably over the lease period, which is included in other liabilities. In accordance with the lease, the Company entered into a cash-collateralized irrevocable standby letter of credit in the amount of $125,345, naming the landlord as beneficiary.

In December 2014, the Company entered into a First Amendment to Lease, or the Amendment, for additional office space contiguous to its current office space in Cambridge, Massachusetts. The Amendment included leasing an additional 8,530 square feet of office space, or the Expansion Space, with an occupancy date of March 13, 2015. The Amendment provided for additional monthly lease payments of approximately $45,000 for the 8,530 square feet for the first twelve months and provided for annual rent escalations thereafter. The monthly rent on the existing 6,837 square feet remained at approximately $32,000 through December 31, 2016, the expiration of the lease. The Amendment included a Landlord’s contribution for leasehold improvements in the amount of approximately $100,000 which was accounted for as a deferred lease incentive and reduction in monthly rent expense over the term of the lease. The Company recorded an additional deferred lease obligation for the Expansion Space which represented the cumulative difference between actual facility lease payments and lease expense recognized ratably over the lease period. The Company had an existing cash-collateralized irrevocable standby letter of credit of $125,345, naming the landlord as beneficiary. In connection with the Amendment, the Company paid an additional cash security deposit to the landlord of $179,130. These amounts are included in other assets.

In November 2015, the Company entered into a Second Amendment to Lease amending the Lease to extend its existing lease for 16,222 square feet on the 11^th floor, which was due to expire on December 31, 2016, and lease an additional 23,189 square feet of office space on the 11^th and 14^th floors. Monthly lease payments for the existing 16,222 square feet on the 11^th floor remain unchanged until December 31, 2016 and will be approximately $107,000 per month commencing on May 1, 2017. The new space leased by the Company under the Second Amendment includes (i) 3,066 square feet on the 14^th floor which was delivered on November 23, 2015 and additional monthly lease payments of approximately $20,000 commencing on March 1, 2016, (ii) 16,739 square feet on the 14^th floor with an estimated delivery date of April 1, 2016 and additional monthly lease payments of approximately $110,000 commencing September 1, 2016 and (iii) 3,384 square feet on the 1th floor with an estimated delivery date of January 1, 2017 and additional monthly lease payments of approximately $23,000 commencing May 1, 2017. The above rents are subject to annual rent escalations, commencing on either February 1, 2017 or February 1, 2018 depending upon the delivery date of each respective suite. The Second Amendment includes a Landlord’s contribution for leasehold improvements for both the new leased space and the Company’s existing premises. The Landlord’s contribution will be accounted for as a deferred lease incentive and reduction in monthly rent expense over the term of the lease. In connection with the Second Amendment, the Company paid an additional cash security deposit to the Landlord of $976,382. The term of the lease, as amended, expires on the later of August 31, 2026 or 10 years after the Landlord delivers the entirety of the 14^th floor to the Company. In connection with the Second Amendment, the Company paid an additional cash security deposit to the landlord of $976,382. Total cash security deposits of $1,280,857 and $304,475 are included in other assets in the Company’s consolidated balance sheets as of December 31, 2015 and 2014, respectively.

The Company leases office equipment under a two three year capital leases with payments commencing in February 2014 and April 2015, respectively. The capital lease amounts are included in accrued expenses and other liabilities.

At December 31, 2015, the Company’s future minimum payments required under these leases are as follows:

	Operating		Capital
	Lease		Lease		Total
	(in thousands)
2016	$	2,522	$	8	$	2,530
2017		3,122		5		3,127
2018		3,122		1		3,123
2019		3,122		—		3,122
2020		3,122		—		3,122
Thereafter		17,687		—		17,687
Total	$	32,697		14	$	32,711
Less amount representing interest				—
Present value of minimum lease payments at December 31, 2015			$	14

The Company recorded approximately $0.9 million and $0.4 million in rent expense for the years ended December 31, 2015 and 2014 respectively.

Under the Company’s agreement with Quintiles to conduct the PRO₂TECT and INNO₂VATE programs, the total cost of committed work not yet paid as of December 31, 2015 was approximately $246.0 million. The estimated period of performance for the committed work with Quintiles is through the third quarter of 2019. The Company contracts with various other organizations to conduct research and development activities with remaining contract costs to the Company of approximately $5.2 million and $4.3 million at December 31, 2015 and December 31, 2014, respectively. The scope of the services under the research and development contracts can be modified and the contracts cancelled by the Company upon written notice. In some instances the contracts may be cancelled by the third party upon written notice.

In September 2015, a purported securities class action lawsuit was filed against the Company, including its Chief Executive Officer, its Chief Financial Officer, and members of its Board of Directors, in the Business Litigation Section of the Suffolk County Superior Court of Massachusetts. The complaint is brought on behalf of an alleged class of those who purchased common stock of the Company pursuant or traceable to our initial public offering, and purports to allege claims arising under Sections 11, 12(a)(2) and 15 of the Securities Act of 1933, as amended (the “Securities Act”). The complaint generally alleges that the defendants violated the federal securities laws by, among other things, making material misstatements or omissions concerning the Phase 2b clinical study of vadadustat. The complaint seeks, among other relief, unspecified compensatory damages, rescission of certain stock purchases, attorneys’ fees, and costs. In October 2015, we removed the case to the United States District Court for the District of Massachusetts, and the plaintiff filed a motion to remand the case back to the Business Litigation Section of the Suffolk County Superior Court of Massachusetts. The plaintiff’s motion to remand is currently pending. The Company believes such claims are without merit, and will engage in a vigorous defense of such litigation.

As explained in more detail below, we have had some positive developments in our opposition and invalidity proceedings against FibroGen, Inc., or FibroGen. With regard to the opposition that we filed in Europe against FibroGen’s European Patent No. 1463823, or the ’823 patent, the European Opposition Division issued a non-binding preliminary opinion that none of FibroGen’s ’823 patent claims meet the requirements for patentability. In an oral proceeding which took place March 8 and 9, 2016, the European Opposition Division confirmed that the patent as granted did not meet the requirements for patentability under the European Patent Convention and, therefore, revoked the patent in its entirety. Likewise, with regard to the invalidity proceeding that we filed in Japan against certain claims of FibroGen’s Japanese Patent No. 4804131, or the ’131 patent, which is the Japanese counterpart to the ’823 patent, the JPO issued a preliminary decision finding all of the challenged claims to be invalid. FibroGen subsequently amended the claims and the JPO accepted the amendments. The resulting FibroGen ’131 patent does not cover vadadustat or any pyridine carboxamide compounds. To date, FibroGen has been unsuccessful in its attempts to obtain a patent in the United States covering the same claim scope as it obtained in Europe and Japan in the ’823 and ’131 patents. In the event FibroGen were to obtain such patents in the United States, we may decide to challenge them like we have done in Europe and Japan. On May 13, May 20, and July 6, 2015, we also filed oppositions to FibroGen’s European Patent Nos. 2322153, 2322155, and 1633333 respectively, requesting the patents be revoked in their entirety.

In June 2013, the European Patent Office granted the ’823 patent, to FibroGen. The ’823 patent claims, among other things, the use of a heterocyclic carboxamide compound selected from the group consisting of pyridine carboxamides, quinoline carboxamides, isoquinoline carboxamides, cinnoline carboxamides, and beta-carboline carboxamides that inhibits HIF-PH enzyme activity in the manufacture of a medicament for increasing endogenous EPO in the prevention, pretreatment or treatment of anemia. On December 5, 2013, we filed an opposition to the ’823 patent requesting that the ’823 patent be revoked in its entirety. The European Opposition Division scheduled oral proceedings for March 8, 2016 and also issued a non-binding preliminary opinion that none of the ’823 patent’s claims met the requirements for patentability. In an oral proceeding which took place March 8 and 9, 2016, the European Opposition Division confirmed that the patent as granted did not meet the requirements for patentability under the European Patent Convention and, therefore, revoked the patent in its entirety. If FibroGen appeals the decision of the European Opposition Division, final resolution of the appeal will likely take two to three years. While, for the reasons set forth in our opposition, we maintain that the ’823 patent should be revoked in its entirety, the ultimate outcome of any appeal remains uncertain. If FibroGen appeals the decision of the European Opposition Division and the Technical Board of Appeal at the European Patent Office decides not to revoke the ’823 patent in its entirety, or only certain claims of the ’823 patent and any surviving claims are determined to encompass our intended use of vadadustat, we may not be able to commercialize vadadustat in the European Union for its intended use, which could materially adversely affect our business, operating results and financial condition.

In August 2011, the Japanese Patent Office granted the ’131 patent, to FibroGen. The ’131 patent claims, among other things, the use of certain heterocyclic carboxamides selected from the group consisting of pyridine carboxamides, quinoline carboxamides, and isoquinoline carboxamides to treat anemia, wherein the heterocyclic carboxamides also suppress HIF prolyl hydroxylase. On June 2, 2014, we filed an invalidity proceeding in the Japanese Patent Office challenging the validity of the ’131 patent and requesting that certain claims be revoked in their entirety. An oral hearing before the Japanese Patent Office was held on February 9, 2015, and on May 11, 2015 the Japanese Patent Office issued a preliminary decision finding all of the challenged claims to be invalid. In response, FibroGen filed a request for correction in which it requested that the ’131 patent claims be amended to exclude pyridine carboxamides from their scope. On November 18, 2015, Akebia received the final trial decision from the JPO in which it accepted FibroGen’s requested claim amendments. As a result of the JPO’s decision and FibroGen’s subsequent amendments, the FibroGen ’131 patent does not cover vadadustat or any pyridine carboxamide compounds.

In August 2014, the European Patent Office granted European Patent Nos. 2322153, 2322155, and 1633333, or the ’153 patent, the ’155 patent, and the ’333 patent, respectively, to FibroGen. These related patents claim, among other things, various compounds that either stabilize HIFα or inhibit a HIF hydroxylase or a HIF prolyl hydroxylase for treating or preventing various conditions, including, inter alia, iron deficiency, microcytosis associated with iron deficiency, anemia of chronic disease, anemia wherein the subject has a transferrin saturation of less than 20%, anemia refractory to treatment with exogenously administered EPO, and microcytosis in microcytic anemia. On May 13, May 20, and July 6, 2015, we filed oppositions to the ’155 patent, the ’333 patent, and the ’153 patent, respectively, requesting that the patents be revoked in their entireties. While, for the reasons set forth in our oppositions, we believe that the ’153 patent, the ’155 patent, and the ’333 patent should be revoked in their entireties, the ultimate outcomes of the oppositions remains uncertain. If the European Patent Office decides not to revoke the ’153 patent, the ’155 patent, or the ’333 patent in their entireties, or only certain claims of those patents, and any surviving claims are determined to encompass our intended use of our lead product candidate, we may not be able to commercialize our lead product candidate in the European Union for its intended use, which could materially adversely affect our business, operating results and financial condition.

The Company is not able to predict the outcome of the lawsuit described above or estimate the amount or range of any possible loss the Company might incur if the Company does not prevail in the final, non-appealable determination of this lawsuit. Therefore, we have not accrued any amounts in connection with this lawsuit.

Employee Retirement Plan	12 Months Ended
Employee Retirement Plan	Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Employee Retirement Plan	9. Employee Retirement Plan During 2008, the Company established a retirement plan (the Plan) authorized by Section 401(k) of the Internal Revenue Code. In accordance with the Plan, all employees who have attained the age of 21 are eligible to participate in the Plan as of the first Entry Date, as defined, following their date of employment. Each employee can contribute a percentage of compensation up to a maximum of the statutory limits per year. Company contributions are discretionary and contributions in the amount of approximately $0.1 million and $0 were made during the year ended December 31, 2015 and 2014, respectively.

Net Loss per Share

12 Months Ended

Dec. 31, 2015

Earnings Per Share [Abstract]

Net Loss per Share

10. Net Loss per Share

The following table presents the calculation of basic and diluted net loss per share applicable to common stockholders:


	Year ended December 31,
	2015			2014
	(in thousands, except share and per share data)
Numerator:
Net loss	$	(60,716	)	$	(37,034	)
Accretion on preferred stock		—			(86,899	)
Net loss applicable to common stockholders	$	(60,716	)	$	(123,933	)

Denominator:
Weighted-average number of common shares – basic and diluted		26,469,170			15,406,386
Net loss per share applicable to common stockholders – basic and diluted	$	(2.29	)	$	(8.04	)

The amounts in the table below were excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, due to their anti-dilutive effect:

	Year ended December 31,
	2015		2014
Outstanding stock options		2,206,635		1,526,346
Unvested restricted stock		216,716		422,145
Unvested restricted stock units		24,425		—
Total		2,447,776		1,948,491

Quarterly Results (unaudited)

12 Months Ended

Dec. 31, 2015

Quarterly Financial Information Disclosure [Abstract]

Quarterly Results (unaudited)

11. Quarterly Results (unaudited)

	Three Months Ended
	March 31,			June 30,			September 30,			December 31,
	2015			2015			2015			2015
	(in thousands, except per share data)
	(unaudited)
Operating expenses	$	10,896		$	10,889		$	19,678		$	20,050
Loss from operations	$	(10,896	)	$	(10,889	)	$	(19,678	)	$	(20,050	)
Other income (expense), net	$	201		$	200		$	203		$	193
Net loss	$	(10,695	)	$	(10,689	)	$	(19,475	)	$	(19,857	)
Net loss per share applicable to common stockholders—basic and diluted	$	(0.53	)	$	(0.40	)	$	(0.68	)	$	(0.66	)


	Three Months Ended
	March 31,			June 30,			September 30,			December 31,
	2014			2014			2014			2014
	(in thousands, except per share data)
	(unaudited)
Operating expenses	$	9,909		$	7,840		$	9,584		$	10,607
Loss from operations	$	(9,909	)	$	(7,840	)	$	(9,584	)	$	(10,607	)
Other income (expense), net	$	212		$	222		$	236		$	237
Net loss	$	(9,697	)	$	(7,618	)	$	(9,348	)	$	(10,370	)
Net loss per share applicable to common stockholders—basic and diluted	$	(43.37	)	$	(0.39	)	$	(0.47	)	$	(0.52	)

Subsequent Event	12 Months Ended
Subsequent Event	Dec. 31, 2015
Subsequent Events [Abstract]
Subsequent Event	12. Subsequent Event In January 2016, in connection with the Company’s Collaboration Agreement which was signed with Mitsubishi Tanabe in December 2015, the Company received a $40.0 million up-front payment. The $40.0 million up-front payment was contingent upon the Company providing certain tax documentation to Mitsubishi which was completed in January 2016. The Company therefore did not have a contractual right to bill until the tax documents were provided in January 2016. Of the $40.0 million received, $20.0 million is subject to refund to Mitsubishi depending on the outcome of discussions with the Japanese regulator about the global trial design.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2015

Accounting Policies [Abstract]

Basis of Presentation

Recent Accounting Pronouncements

Segment Information

Use of Estimates

Cash and Cash Equivalents

Investments

Patents

Costs incurred in connection with the application for and issuance of patents are expensed as incurred.

Income Taxes

Stock-Based Compensation

Fair Value of Financial Instruments

Level 1 – Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

Level 2 – Valuations based on quoted prices for similar assets or liabilities in markets that are not active, or for which all significant inputs are observable, either directly or indirectly.

Level 3 – Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable.

Concentrations of Credit Risk and Off-Balance Sheet Risk

Net Loss per Share

Property and Equipment

The following is the summary of property and equipment and related accumulated depreciation as of December 31, 2015 and 2014.

	Useful Life	2015			2014
		(in thousands)
Computer equipment and software	3	$	300		$	99
Furniture and fixtures	5		243			117
Equipment	7		50			6
Leasehold improvements	Shorter of the useful life or remaining lease term (3 years)		70			27
Office equipment under capital lease	3		24			12
			687			261
Less accumulated depreciation			(147	)		(51	)
Net property and equipment		$	540		$	210

Depreciation expense, including expense associated with assets under capital leases, was approximately $96,000, $49,000 and $1,000 for the years ended December 31, 2015, 2014 and 2013, respectively.

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2015

Accounting Policies [Abstract]

Summary of Equipment and Related Accumulated Depreciation

The following is the summary of property and equipment and related accumulated depreciation as of December 31, 2015 and 2014.

	Useful Life	2015			2014
		(in thousands)
Computer equipment and software	3	$	300		$	99
Furniture and fixtures	5		243			117
Equipment	7		50			6
Leasehold improvements	Shorter of the useful life or remaining lease term (3 years)		70			27
Office equipment under capital lease	3		24			12
			687			261
Less accumulated depreciation			(147	)		(51	)
Net property and equipment		$	540		$	210

Available for Sale Securities (Tables)

12 Months Ended

Dec. 31, 2015

Investments Debt And Equity Securities [Abstract]

Summary of Available for Sale Securities at Fair Value

Available for sale securities at December 31, 2015 and 2014 consist of the following:

			Gross		Gross
			Unrealized		Unrealized
	Amortized Cost		Gains		Losses			Fair Value
	(in thousands)
December 31, 2015
Cash and cash equivalents	$	49,778	$	—	$	—		$	49,778
Available for sale securities:
Certificates of deposit	$	21,505	—		—			$	21,505
U.S. Government debt securities		46,461	—			(185	)		46,276
Corporate debt securities		20,944		1		(50	)		20,895
Total available for sale securities	$	88,910	$	1	$	(235	)	$	88,676
Total cash, cash equivalents, and available for sale securities	$	138,688	$	1	$	(235	)	$	138,454

			Gross		Gross
			Unrealized		Unrealized
	Amortized Cost		Gains		Losses			Fair Value
	(in thousands)
December 31, 2014
Cash and cash equivalents	$	32,780	$	—	$	—		$	32,780
Available for sale securities:
Certificates of deposit	$	13,429	—			—		$	13,429
U.S. Government debt securities		38,412		1		(28	)		38,385
Commercial paper		2,499		—		—			2,499
Corporate debt securities		21,854		3		(32	)		21,825
Total available for sale securities	$	76,194	$	4	$	(60	)	$	76,138
Total cash, cash equivalents, and available for sale securities	$	108,974	$	4	$	(60	)	$	108,918

Estimated Fair Value of Available for Sale Securities at Fair Value by Contractual Maturity

The estimated fair value of the Company’s available-for-sale securities balance at December 31, 2015, by contractual maturity, is as follows:

Due in one year or less	$	39,075
Due after one year		49,601
Total available for sale securities	$	88,676

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2015

Fair Value Disclosures [Abstract]

Assets Measured or Disclosed at Fair Value on Recurring Basis

Assets measured or disclosed at fair value on a recurring basis as of December 31, 2015 are summarized below:

	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets:
Cash and cash equivalents	$	49,778	$	—	$	—	$	49,778
Certificates of deposit		—		21,505		—		21,505
U.S. Government debt securities		—		46,276		—		46,276
Corporate debt securities		—		20,895		—		20,895
	$	49,778	$	88,676	$	—	$	138,454

Assets measured or disclosed at fair value on a recurring basis as of December 31, 2014 are summarized below:

	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets:
Cash and cash equivalents	$	32,780	$	—	$	—	$	32,780
Certificates of deposit		—		13,429		—		13,429
U.S. Government debt securities		—		38,385		—		38,385
Commercial paper		—		2,499		—		2,499
Corporate debt securities		—		21,825		—		21,825
	$	32,780	$	76,138	$	—	$	108,918

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2015

Payables And Accruals [Abstract]

Components of Accrued Expenses

Accrued expenses are as follows:

	December 31,
	2015		2014
	(in thousands)
Accrued clinical	$	4,536	$	2,132
Accrued bonus		2,178		1,286
Professional fees		647		328
Accrued payroll		518		213
Accrued vacation		310		177
Accrued severance	—			179
Other		1,366		549
Total accrued expenses	$	9,555	$	4,864

Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2015

Summary of Reserved Shares of Common Stock for Future Issuance

The Company has reserved for future issuance the following number of shares of common stock:

	December 31,
	2015		2014
Unvested restricted common stock		216,716		422,145
Common stock options and RSU's outstanding		2,231,057		1,526,346
Shares available for issuance under the 2014 Plan ⁽¹⁾		1,318,732		1,549,154
Shares available for issuance under the ESPP		440,304		262,500
Total		4,206,809		3,760,145

⁽¹⁾

On January 1, 2016, the shares reserved for future grants under the 2014 Plan increased by 986,800 shares pursuant to the 2014 Plan Evergreen Provision.

⁽²⁾

On February 28, 2016, the shares reserved for future issuance under the ESPP increased by 379,430 shares pursuant to the ESPP Evergreen Provision.

Schedule of Assumptions Used in Black-Scholes Option Pricing Model

The assumptions used in the Black-Scholes pricing model to estimate the grant date fair value of options granted to employees are as follows:

	Year ended December 31,
	2015		2014
Risk-free interest rate	1.44% - 1.95%		1.63% - 2.06%
Dividend yield		0.00%		0.00%
Volatility	62.47% - 81.25%		67.97% - 77.00%
Expected term (years)	5.51 - 6.25		6.25

Schedule of Stock Option Activity

The following table summarizes the Company’s stock option activity:

	Shares			Weighted-Average Exercise Price		Weighted-Average Contractual Life (in years)		Aggregate Intrinsic Value
Outstanding, December 31, 2014		1,526,349		$	7.57			$	11,523,531
Granted		992,050		$	9.81
Exercised		(218,674	)	$	0.60			$	2,047,507
Forfeited		(93,090	)	$	12.96			$	107,384
Expired/cancelled		—
Outstanding, December 31, 2015		2,206,635		$	9.04		8.20	$	13,114,811
Options exercisable, December 31, 2015		610,696		$	6.20		6.63	$	5,620,564
Expected to vest, December 31, 2015		1,958,808		$	9.71		8.56	$	10,737,600

Schedule of Restricted Stock Activity

A summary of the Company’s restricted stock activity is as follows:

	Shares			Weighted-Average Grant Date Fair Value
Unvested balance, December 31, 2014		422,145		$	8.39
Granted		-
Vested		(169,376	)	$	8.87
Forfeited		(36,053	)	$	8.93
Outstanding, December 31, 2015		216,716		$	7.97

Compensation Cost Related to Share-Based Awards

Compensation Expense Summary

The Company has recognized the following compensation cost related to share-based awards:


	Year ended December 31,
	2015		2014
	(in thousands)
Research and development	$	2,079	$	2,766
General and administrative		2,635		3,244
Total	$	4,714	$	6,010

Schedule of Compensation Expense by Type of Award


	Year ended December 31,
	2015		2014
	(in thousands)
Stock options	$	3,660	$	1,925
Restricted stock		909		4,085
Restricted stock units		62		—
Employee stock purchase plan		83		—
Total	$	4,714	$	6,010

Restricted Stock Units

Schedule of Restricted Stock Activity

	Shares			Weighted-Average Grant Date Fair Value
Unvested balance, December 31, 2014		-		$	-
Granted		27,875		$	11.15
Vested		-		$	-
Forfeited		(3,450	)	$	11.15
Outstanding, December 31, 2015		24,425		$	11.15

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2015

Income Tax Disclosure [Abstract]

Schedule of Reconciliation of Statutory Federal Income Tax Rate to Effective Tax Rate

Following is a reconciliation of the statutory federal income rate to the Company’s effective tax rate:

	Year ended December 31,
	2015			2014
Federal tax at statutory rate		34.0	%		34.0	%
State and local tax at statutory rate		3.0			5.5
Research and development tax credits		0.4			(0.6	)
Equity compensation		(0.6	)		(1.6	)
Other permanent differences and credits		—			(0.1	)
Change in valuation allowance		(36.8	)		(37.2	)
Effective tax rate		0.0	%		0.0	%

Schedule of Significant Deferred Tax Assets and Liabilities

Significant components of the Company’s deferred tax assets and liabilities are as follows:

		December 31,
		2015			2014
	(in thousands)
Deferred tax assets:
Accrued expenses		$	912		$	559
Intangible assets			527		$	603
Restricted stock			301		$	436
Fixed assets			—		$	2
Non-qualified stock options ⁽¹⁾			1,558		$	440
Research and development credits			1,829		$	1,589
Net operating loss carryforward			51,356		$	30,559
Other			64		$	39
Total deferred tax assets			56,547		$	34,227
Less valuation allowance			(56,545	)		(34,227	)
Total deferred tax assets, net of valuation allowance			2			—
Deferred tax liabilities:
Fixed assets			(2	)		—
Restricted stock			—			—
Total deferred tax liabilities			(2	)		—
Net deferred tax asset		$	—		$	—

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2015

Commitments And Contingencies Disclosure [Abstract]

Schedule of Future Minimum Payments Required Under Leases

At December 31, 2015, the Company’s future minimum payments required under these leases are as follows:

	Operating		Capital
	Lease		Lease		Total
	(in thousands)
2016	$	2,522	$	8	$	2,530
2017		3,122		5		3,127
2018		3,122		1		3,123
2019		3,122		—		3,122
2020		3,122		—		3,122
Thereafter		17,687		—		17,687
Total	$	32,697		14	$	32,711
Less amount representing interest				—
Present value of minimum lease payments at December 31, 2015			$	14

- Definition

Tabular disclosure of a lessee's leasing arrangements including: (1) the basis on which contingent rental payments are determined, (2) the existence and terms of renewal or purchase options and escalation clauses, (3) restrictions imposed by lease arrangements, such as those concerning dividends, additional debt, and further leasing, (4) rent holidays, rent concessions, or leasehold improvement incentives and unusual provisions or conditions. Disclosure may also include the specific period used to amortize material leasehold improvements made at the inception of the lease or during the lease term. Additionally, for operating leases having initial or remaining noncancelable lease terms in excess of one year: (a) future minimum rental payments required as of the date of the latest balance sheet presented, in the aggregate and for each of the five succeeding fiscal years, (b) the total of minimum rentals to be received in the future under noncancelable subleases as of the date of the latest balance sheet presented, and (c) for all operating leases, rental expense for each period for which an income statement is presented, with separate amounts for minimum rentals, contingent rentals, and sublease rentals. Rental payments under leases with terms of a month or less that were not renewed need not be included.

+ References + Details

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2015

Earnings Per Share [Abstract]

Calculation of Basic and Diluted Net Loss per Share

The following table presents the calculation of basic and diluted net loss per share applicable to common stockholders:


	Year ended December 31,
	2015			2014
	(in thousands, except share and per share data)
Numerator:
Net loss	$	(60,716	)	$	(37,034	)
Accretion on preferred stock		—			(86,899	)
Net loss applicable to common stockholders	$	(60,716	)	$	(123,933	)

Denominator:
Weighted-average number of common shares – basic and diluted		26,469,170			15,406,386
Net loss per share applicable to common stockholders – basic and diluted	$	(2.29	)	$	(8.04	)

Anti-Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share

The amounts in the table below were excluded from the calculation of diluted net loss per share, prior to the use of the treasury stock method, due to their anti-dilutive effect:

	Year ended December 31,
	2015		2014
Outstanding stock options		2,206,635		1,526,346
Unvested restricted stock		216,716		422,145
Unvested restricted stock units		24,425		—
Total		2,447,776		1,948,491

Quarterly Results (unaudited) (Tables)

12 Months Ended

Dec. 31, 2015

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Results

	Three Months Ended
	March 31,			June 30,			September 30,			December 31,
	2015			2015			2015			2015
	(in thousands, except per share data)
	(unaudited)
Operating expenses	$	10,896		$	10,889		$	19,678		$	20,050
Loss from operations	$	(10,896	)	$	(10,889	)	$	(19,678	)	$	(20,050	)
Other income (expense), net	$	201		$	200		$	203		$	193
Net loss	$	(10,695	)	$	(10,689	)	$	(19,475	)	$	(19,857	)
Net loss per share applicable to common stockholders—basic and diluted	$	(0.53	)	$	(0.40	)	$	(0.68	)	$	(0.66	)


	Three Months Ended
	March 31,			June 30,			September 30,			December 31,
	2014			2014			2014			2014
	(in thousands, except per share data)
	(unaudited)
Operating expenses	$	9,909		$	7,840		$	9,584		$	10,607
Loss from operations	$	(9,909	)	$	(7,840	)	$	(9,584	)	$	(10,607	)
Other income (expense), net	$	212		$	222		$	236		$	237
Net loss	$	(9,697	)	$	(7,618	)	$	(9,348	)	$	(10,370	)
Net loss per share applicable to common stockholders—basic and diluted	$	(43.37	)	$	(0.39	)	$	(0.47	)	$	(0.52	)

Nature of Organization and Operations - Additional Information (Details)			1 Months Ended			12 Months Ended
	Mar. 25, 2014 USD ($) $ / shares shares	Mar. 06, 2014	Jan. 31, 2016 USD ($) $ / shares shares	Aug. 31, 2015 USD ($)	Apr. 30, 2015 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) Patient shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2013 shares
Nature Of Organization And Operations [Line Items]
Convertible redeemable preferred stock converted into common stock \| shares	12,115,183						0
Common stock, shares authorized \| shares	175,000,000					175,000,000	175,000,000
Preferred stock, shares authorized \| shares	25,000,000					25,000,000	25,000,000
Cash and cash equivalents and available for sale securities						$ 138,454,000	$ 108,918,000
Stock split, conversion ratio		1.75
Entity incorporation date						Feb. 27, 2007
Cantor Fitzgerald & Co
Nature Of Organization And Operations [Line Items]
Issuance of common stock, shares \| shares						1,719,434
Proceeds from sale of common stock, net of commissions						$ 18,400,000
Issuance of additional common stock, shares \| shares						0
IPO
Nature Of Organization And Operations [Line Items]
Sale of stock, price per share \| $ / shares	$ 17.00
Proceeds from issuance of initial public offering	$ 104,400,000
Convertible redeemable preferred stock converted into common stock \| shares	12,115,183
Follow-on public offering
Nature Of Organization And Operations [Line Items]
Sale of stock, price per share \| $ / shares					$ 8.25
Proceeds from issuance of initial public offering					$ 64,600,000
Follow-on public offering \| Subsequent Event
Nature Of Organization And Operations [Line Items]
Sale of stock, price per share \| $ / shares			$ 9.00
Proceeds from issuance of initial public offering			$ 61,000,000
Common Stock
Nature Of Organization And Operations [Line Items]
Issuance of common stock, shares \| shares						10,083,070	6,762,000	176,717
Common Stock \| IPO
Nature Of Organization And Operations [Line Items]
Issuance of common stock, shares \| shares	6,762,000
Common Stock \| Overallotment option granted to underwriters
Nature Of Organization And Operations [Line Items]
Issuance of common stock, shares \| shares	879,647				1,090,909
Common Stock \| Follow-on public offering
Nature Of Organization And Operations [Line Items]
Issuance of common stock, shares \| shares					8,363,636
Common Stock \| Follow-on public offering \| Subsequent Event
Nature Of Organization And Operations [Line Items]
Issuance of common stock, shares \| shares			7,250,000
Maximum \| Cantor Fitzgerald & Co
Nature Of Organization And Operations [Line Items]
Common stock sales agreement amount				$ 50,000,000
Phase 3 vadadustat program \| Minimum
Nature Of Organization And Operations [Line Items]
Cost of program per patient						$ 80,000
Phase 3 vadadustat program \| Maximum
Nature Of Organization And Operations [Line Items]
Cost of program per patient						$ 85,000
PRO2TECT
Nature Of Organization And Operations [Line Items]
Number of patients \| Patient						3,100
INNO2VATE
Nature Of Organization And Operations [Line Items]
Number of patients \| Patient						2,600
Development and Commercialize Collaboration Agreement
Nature Of Organization And Operations [Line Items]
Collaborative arrangement right description						In December 2015, we entered into a collaboration agreement with Mitsubishi Tanabe to develop and commercialize vadadustat in Japan and certain other countries in Asia for total milestone payments of up to $350 million, including up to $100 million in upfront and development payments, of which $40.0 million was received in January 2016. Of the $40.0 million received, $20.0 million is subject to refund to Mitsubishi depending on the outcome of discussions with the Japanese regulator about the global trial design. In addition, we will receive tiered double-digit royalty payments on sales of vadadustat.
Development and Commercialize Collaboration Agreement \| Subsequent Event \| Upfront
Nature Of Organization And Operations [Line Items]
Milestone revenue			$ 40,000,000
Milestone revenue refundable			$ 20,000,000
Development and Commercialize Collaboration Agreement \| Maximum
Nature Of Organization And Operations [Line Items]
Milestone revenue						$ 350,000,000
Development and Commercialize Collaboration Agreement \| Maximum \| Upfront
Nature Of Organization And Operations [Line Items]
Milestone revenue						$ 100,000,000

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) Segment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments \| Segment	1
Significant uncertain tax positions	$ 0	$ 0
Revenue	$ 0
Expected dividend yield	0.00%
Depreciation expense	$ 96,000	$ 49,000	$ 1,000
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, Useful life	3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, Useful life	7 years

Summary of Significant Accounting Policies - Summary of Property and Equipment and Related Accumulated Depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Gross property, plant and equipment	$ 687	$ 261
Less accumulated depreciation	(147)	(51)
Net property and equipment	$ 540	210
Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful life	7 years
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful life	3 years
Gross property, plant and equipment	$ 300	99
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful life	5 years
Gross property, plant and equipment	$ 243	117
Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful life	7 years
Gross property, plant and equipment	$ 50	6
Leasehold improvements
Property, Plant and Equipment [Line Items]
Leasehold improvements, Useful Life	Shorter of the useful life or remaining lease term (3 years)
Gross property, plant and equipment	$ 70	27
Leasehold improvements \| Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful life	3 years
Office equipment under capital lease
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Useful life	3 years
Gross property, plant and equipment	$ 24	$ 12

Available for Sale Securities - Summary of Available for Sale Securities at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 88,910	$ 76,194
Gross Unrealized Gains	1	4
Gross Unrealized Losses	(235)	(60)
Fair Value	88,676	76,138
Cash and cash equivalents	49,778	32,780	$ 21,215	$ 1,641
Cash and cash equivalents, fair value	49,778	32,780
Total cash, cash equivalents, and available for sale securities - Amortized Cost	138,688	108,974
Cash and cash equivalents and available for sale securities	138,454	108,918
U.S. Government debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	46,461	38,412
Gross Unrealized Gains		1
Gross Unrealized Losses	(185)	(28)
Fair Value	46,276	38,385
Certificates of deposit
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	21,505	13,429
Fair Value	21,505	13,429
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		2,499
Fair Value		2,499
Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	20,944	21,854
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(50)	(32)
Fair Value	$ 20,895	$ 21,825

Available for Sale Securities - Estimated Fair Value of Available for Sale Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Investments Debt And Equity Securities [Abstract]
Due in one year or less	$ 39,075
Due after one year	49,601
Total available for sale securities	$ 88,676	$ 76,138

Fair Value of Financial Instruments - Assets Measured or Disclosed at Fair Value on Recurring Basis (Detail) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 0	$ 0
Fair value measurements recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	138,454,000	108,918,000
Fair value measurements recurring \| U.S. Government debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	46,276,000	38,385,000
Fair value measurements recurring \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	49,778,000	32,780,000
Fair value measurements recurring \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	88,676,000	76,138,000
Fair value measurements recurring \| Level 2 \| U.S. Government debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	46,276,000	38,385,000
Fair value measurements recurring \| Cash and cash equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	49,778,000	32,780,000
Fair value measurements recurring \| Cash and cash equivalents \| Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	49,778,000	32,780,000
Fair value measurements recurring \| Certificates of deposit
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	21,505,000	13,429,000
Fair value measurements recurring \| Certificates of deposit \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	21,505,000	13,429,000
Fair value measurements recurring \| Corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	20,895,000	21,825,000
Fair value measurements recurring \| Corporate debt securities \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 20,895,000	21,825,000
Fair value measurements recurring \| Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis		2,499,000
Fair value measurements recurring \| Commercial Paper \| Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis		$ 2,499,000

Fair Value of Financial Instruments - Additional Information (Detail) - Level 3 - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 0	$ 0
Liabilities measured at fair value on recurring basis	$ 0	$ 0

Accrued Expenses - Components of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Payables And Accruals [Abstract]
Accrued clinical	$ 4,536	$ 2,132
Accrued bonus	2,178	1,286
Professional fees	647	328
Accrued payroll	518	213
Accrued vacation	310	177
Accrued severance		179
Other	1,366	549
Total accrued expenses	$ 9,555	$ 4,864

Accrued Expenses - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014
Accrued Expenses [Line Items]
Accrued severance			$ 179,000
Separation Agreement 1
Accrued Expenses [Line Items]
Severance expense	$ 300,000
Payment out of severance accrual			300,000
Accrued severance		$ 0	45,890
Separation Agreement 2
Accrued Expenses [Line Items]
Severance expense		300,000	100,000
Accrued severance		$ 0	$ 100,000
Severance agreement description		In August 2014, the Company entered into a separation agreement with an employee, which became effective on August 13, 2014. The Company will record the expense and liability associated with the separation agreement ratably over the period from August 5, 2014 through December 31, 2015 because the severance payments are subject to continued service and forfeiture until December 31, 2015. During the year ended 2015 and 2014, the Company recorded severance expense in the amount of approximately $0.3 million and $0.1 million respectively, which was recorded to research and development expense. At December 31, 2015 and 2014, approximately $0 and $0.1 million, respectively, remained in accrued expense in relation to the unpaid severance costs.

Stockholders' Equity - Additional Information (Details)					1 Months Ended		12 Months Ended
Stockholders' Equity - Additional Information (Details)	Mar. 06, 2015 shares	Mar. 25, 2014 USD ($) shares	Mar. 06, 2014	Dec. 23, 2013 USD ($) $ / shares shares	Dec. 31, 2013 $ / shares shares	Sep. 30, 2013 $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2013 USD ($) shares	Feb. 28, 2014 shares	Dec. 31, 2012 shares
Class of Stock [Line Items]
Common stock, shares authorized		175,000,000					175,000,000	175,000,000
Common stock, par value \| $ / shares							$ 0.00001	$ 0.00001
Common stock, shares issued							30,662,218	20,370,624
Common stock, shares outstanding							30,662,218	20,370,624
Preferred stock, shares authorized		25,000,000					25,000,000	25,000,000
Preferred stock, par value \| $ / shares							$ 0.00001	$ 0.00001
Preferred stock, shares issued							0	0
Preferred stock, shares outstanding							0	0
Stock split, conversion ratio			1.75
Convertible redeemable preferred stock converted into common stock		12,115,183						0
Common stock, shares reserved for future issuance							4,206,809	3,760,145
Compensation not yet recognized, stock options \| $							$ 9,700,000
Weighted average period of recognition for unrecognized compensation costs							2 years 7 months 13 days
Stock-based compensation expense \| $							$ 4,714,000	$ 6,010,000	$ 1,564,000
Employee Severance
Class of Stock [Line Items]
Stock-based compensation expense \| $								$ 1,000,000
Common Stock
Class of Stock [Line Items]
Common stock, shares outstanding					1,383,345		30,662,218	20,370,624	1,383,345		615,757
Total compensation cost recognized \| $									$ 1,100,000
Former Employees \| Common Stock
Class of Stock [Line Items]
Granted, Shares					105,753	70,964
Granted, Weighted-Average Grant Date Fair Value \| $ / shares					$ 7.42	$ 3.77
Restricted Stock Units
Class of Stock [Line Items]
Granted, Shares							27,875
Vesting periods							3 years
Weighted average period of recognition for unrecognized compensation costs							2 years 2 months 5 days
Granted, Weighted-Average Grant Date Fair Value \| $ / shares							$ 11.15
Stock-based compensation expense \| $							$ 62,000
Unrecognized compensation cost \| $							200,000
Total compensation cost to be recognized \| $							$ 300,000
Restricted Stock Units \| Share Based Compensation Award Tranche One
Class of Stock [Line Items]
Vesting percentage							100.00%
Stock Options
Class of Stock [Line Items]
Shares issued	479,750
Stock-based compensation description							Options vest in installments of (i) 25% at the one year anniversary and (ii) in either 36 or 48 equal monthly or 12 equal quarterly installments beginning in the thirteenth month after the initial Vesting Commencement Date (as defined) or grant date, subject to the employee’s continuous service with the Company
Options expiration after the date of grant							10 years
Stock-based compensation expense \| $							$ 3,660,000	$ 1,925,000
Stock Options \| Share Based Compensation Award Tranche One
Class of Stock [Line Items]
Vesting percentage							25.00%
Restricted Stock
Class of Stock [Line Items]
Common stock, shares reserved for future issuance							216,716	422,145
Vesting periods				1 year
Weighted average period of recognition for unrecognized compensation costs							1 year 29 days
Granted, Weighted-Average Grant Date Fair Value \| $ / shares				$ 7.42
Restricted stock requisite service period description							The awards of restricted stock contained a performance condition wherein vesting is contingent upon the Company’s consummation of a liquidity event, as defined, prior to the fifth anniversary of the date of grant. Certain of the awards of restricted stock have a requisite service period that was complete upon grant. The remainder of the awards of restricted stock have a requisite service period of four years whereby the award vests 25% on the one year anniversary of the Vesting Commencement Date (as defined), then ratably on the first day of each calendar quarter for 12 quarters, subject to continuous service by the individual and achievement of the performance target
Grant-date fair value of restricted stock issued \| $				$ 3,900,000
Award requisite service period				4 years
Stock-based compensation expense \| $		$ 100,000					$ 909,000	$ 4,085,000
Unrecognized compensation cost \| $							$ 400,000
Restricted Stock \| Share Based Compensation Award Tranche One
Class of Stock [Line Items]
Vesting percentage				25.00%
Restricted Stock \| Employees
Class of Stock [Line Items]
Granted, Shares				450,224
Restricted Stock \| Non Employees
Class of Stock [Line Items]
Granted, Shares				79,067
Maximum \| Stock Options
Class of Stock [Line Items]
Vesting periods							48 months
Maximum \| Stock Options \| Director
Class of Stock [Line Items]
Vesting periods							48 months
Maximum \| Non Employee Stock Option
Class of Stock [Line Items]
Vesting periods							48 months
Minimum \| Stock Options
Class of Stock [Line Items]
Vesting periods							12 months
Minimum \| Stock Options \| Director
Class of Stock [Line Items]
Vesting periods							12 months
Minimum \| Non Employee Stock Option
Class of Stock [Line Items]
Vesting periods							12 months
2014 Plans
Class of Stock [Line Items]
Common stock, shares reserved for future issuance							1,318,732	1,549,154		1,785,000
Incremental rate at which the shares reserved for issuance increase							3.00%
2014 Plans \| Employees
Class of Stock [Line Items]
Granted, Shares							957,050
2014 Plans \| Director
Class of Stock [Line Items]
Granted, Shares							35,000
2014 Plans \| Restricted Stock Units
Class of Stock [Line Items]
Granted, Shares							27,875
2014 Plans \| Maximum
Class of Stock [Line Items]
Common stock, shares reserved for future issuance							1,131,937
2014 Employee Stock Purchase Plan
Class of Stock [Line Items]
Maximum shares of common stock authorized to be issued							262,500
Percentage of increase on common stock authorized on each anniversary							1.00%
Number of common stock outstanding							739,611
Common stock shares authorized percentage under the plan							5.00%
Purchase price at the end of offering period							85.00%
Offering period							6 months
Shares issued							25,903
Stock-based compensation expense \| $							$ 83,000

Stockholders' Equity - Summary of Reserved Shares of Common Stock for Future Issuance (Details) - shares	Dec. 31, 2015	Dec. 31, 2014	Feb. 28, 2014
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	4,206,809	3,760,145
Restricted Stock
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	216,716	422,145
Stock Options and Restricted Stock Units
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	2,231,057	1,526,346
Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	440,304	262,500
2014 Plans
Class of Stock [Line Items]
Common stock, shares reserved for future issuance	1,318,732	1,549,154	1,785,000

Stockholders' Equity - Schedule of Assumptions Used in Black-Scholes Option Pricing Model (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%	0.00%
Expected term (years)		6 years 3 months
Stock Options \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.44%	1.63%
Volatility	62.47%	67.97%
Expected term (years)	5 years 6 months 4 days
Stock Options \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.95%	2.06%
Volatility	81.25%	77.00%
Expected term (years)	6 years 3 months

Stockholders' Equity - Schedule of Stock Option Activity (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) $ / shares shares
Share under option
Outstanding beginning balance, Shares \| shares	1,526,349
Granted, Shares \| shares	992,050
Exercised, Shares \| shares	(218,674)
Forfeited, Shares \| shares	(93,090)
Outstanding ending balance, Shares \| shares	2,206,635
Options exercisable at end of period, Shares \| shares	610,696
Expected to vest at end of period, Shares \| shares	1,958,808
Weighted Average-Exercise Price
Outstanding beginning balance, Weighted-Average Exercise Price \| $ / shares	$ 7.57
Granted, Weighted-Average Exercise Price \| $ / shares	9.81
Exercised, Weighted-Average Exercise Price \| $ / shares	0.60
Forfeited, Weighted-Average Exercise Price \| $ / shares	12.96
Outstanding ending balance, Weighted-Average Exercise Price \| $ / shares	9.04
Options exercisable at end of period, Weighted-Average Exercise Price \| $ / shares	6.20
Expected to vest at end of period, Weighted-Average Exercise Price \| $ / shares	$ 9.71
Weighted-Average Remaining Contractual Term
Outstanding ending balance, Weighted-Average Contractual Life(in years)	8 years 2 months 12 days
Options exercisable at end of period, Weighted-Average Contractual Life(in years)	6 years 7 months 17 days
Expected to vest at end of period, Weighted-Average Contractual Life(in years)	8 years 6 months 22 days
Aggregate Intrinsic Value
Outstanding beginning balance, Aggregate Intrinsic Value \| $	$ 11,523,531
Exercised, Aggregate Intrinsic Value \| $	2,047,507
Forfeited, Aggregate Intrinsic Value \| $	107,384
Outstanding ending balance, Aggregate Intrinsic Value \| $	13,114,811
Options exercisable at end of period, Aggregate Intrinsic Value \| $	5,620,564
Expected to vest at end of period, Aggregate Intrinsic Value \| $	$ 10,737,600

Stockholders' Equity - Summary of Company Restricted Stock Activity (Details) - $ / shares		12 Months Ended
	Dec. 23, 2013	Dec. 31, 2015
Restricted Stock
Restricted stock activity, Shares
Unvested beginning balance, Shares		422,145
Vested, Shares		(169,376)
Forfeited, Shares		(36,053)
Unvested ending balance, Shares		216,716
Unvested beginning balance, Weighted-Average Grant Date Fair Value		$ 8.39
Granted, Weighted-Average Grant Date Fair Value	$ 7.42
Vested, Weighted-Average Grant Date Fair Value		8.87
Forfeited, Weighted-Average Grant Date Fair Value		8.93
Unvested ending balance, Weighted-Average Grant Date Fair Value		$ 7.97
Restricted Stock Units
Restricted stock activity, Shares
Granted, Shares		27,875
Forfeited, Shares		(3,450)
Unvested ending balance, Shares		24,425
Granted, Weighted-Average Grant Date Fair Value		$ 11.15
Forfeited, Weighted-Average Grant Date Fair Value		11.15
Unvested ending balance, Weighted-Average Grant Date Fair Value		$ 11.15

Stockholders Equity - Compensation Cost Related to Share-Based Awards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 4,714	$ 6,010	$ 1,564
Research and Development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	2,079	2,766
General and Administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 2,635	$ 3,244

Stockholders Equity - Schedule of Compensation Expense by Type of Award (Details) - USD ($)		12 Months Ended
	Mar. 25, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		$ 4,714,000	$ 6,010,000	$ 1,564,000
Stock Options
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		3,660,000	1,925,000
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 100,000	909,000	$ 4,085,000
Restricted Stock Units
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		62,000
Employee Stock Purchase Plan
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense		$ 83,000

Income Taxes - Schedule of Reconciliation of Statutory Federal Income Tax Rate to Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Federal tax at statutory rate	34.00%	34.00%
State and local tax at statutory rate	3.00%	5.50%
Research and development tax credits	0.40%	(0.60%)
Equity compensation	(0.60%)	(1.60%)
Other permanent differences and credits		(0.10%)
Change in valuation allowance	(36.80%)	(37.20%)
Effective tax rate	0.00%	0.00%

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Line Items]
Increase in valuation allowance, deferred tax assets	$ 22,300,000	$ 13,800,000
Deferred capitalized start up expense net of amortization	1,000,000	1,200,000
Deferred income tax amortization	900,000	800,000
Net operation loss carryforwards deduction related to equity compensation	$ 900,000	900,000
Changes in ownership interest rate period	3 years
Cumulative changes in ownership interest of shareholders	50.00%
Unrecognized tax benefits	$ 0	0	$ 0
Income tax penalties and interest accrued associated with unrecognized tax benefits	0	0	0
Income tax penalties and interest expense associated with unrecognized tax benefits	0	0	$ 0
Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	140,500,000	84,200,000
State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 111,900,000	$ 83,700,000
Operating Loss Carryforwards Expiration Year	2035
Federal and State \| Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 2,000,000
Earliest Tax Year
Income Tax Disclosure [Line Items]
NOL and R&D tax credit carry forward expiration year	2027
Latest Tax Year
Income Tax Disclosure [Line Items]
NOL and R&D tax credit carry forward expiration year	2035

Income Taxes -Schedule of Significant Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Accrued expenses	$ 912	$ 559
Intangible assets	527	603
Restricted stock	301	436
Fixed assets		2
Non-qualified stock options	1,558	440
Research and development credits	1,829	1,589
Net operating loss carryforward	51,356	30,559
Other	64	39
Total deferred tax assets	56,547	34,227
Less valuation allowance	(56,545)	(34,227)
Total deferred tax assets, net of valuation allowance	2
Deferred tax liabilities:
Fixed assets	(2)
Total deferred tax liabilities	(2)
Net deferred tax asset	$ 0	$ 0

Commitments and Contingencies - Additional Information (Details)	1 Months Ended			12 Months Ended
	Nov. 30, 2015 USD ($) ft²	Dec. 31, 2014 USD ($) ft²	Dec. 31, 2013 USD ($) ft²	Dec. 31, 2015 USD ($) ft²	Dec. 31, 2014 USD ($) ft²
Commitments And Contingencies [Line Items]
Lease period			3 years
Area of property leased \| ft²		8,530	6,837		8,530
Monthly lease payments		$ 45,000	$ 31,000	$ 32,000
Rent abatement			$ 31,000
Lease commenced and rental payments period			2014-01
Cash-collateralized irrevocable standby letter of credit			$ 125,345	$ 125,345
Lease space occupancy date		Mar. 13, 2015
Lease expiration date				Dec. 31, 2016
Landlord contribution for leasehold improvements		$ 100,000			$ 100,000
Additional cash security deposit paid to landlord		179,130			179,130
Rent expense associated with lease				$ 900,000	400,000
Contract cost incurred in research and development activities				5,200,000	4,300,000
Total cost of committed work not yet paid				$ 246,000,000
Office Equipment One
Commitments And Contingencies [Line Items]
Capital leases term				2 years
Capital lease commencement period				2014-02
Office Equipment Two
Commitments And Contingencies [Line Items]
Capital leases term				3 years
Capital lease commencement period				2015-04
Second Amendment
Commitments And Contingencies [Line Items]
Lease period				10 years
Area of property leased \| ft²	23,189
Monthly lease payments	$ 107,000
Lease expiration date				Aug. 31, 2026
Additional cash security deposit paid to landlord	$ 976,382
Square feet of leased property \| ft²	16,222
Operating lease payments commencement date				May 01, 2017
Second Amendment \| Other Assets
Commitments And Contingencies [Line Items]
Additional cash security deposit paid to landlord		$ 304,475		$ 1,280,857	$ 304,475
Second Amendment \| Delivered on November 23, 2015
Commitments And Contingencies [Line Items]
Monthly lease payments				$ 20,000
Square feet of leased property \| ft²				3,066
Operating lease payments commencement date				Mar. 01, 2016
Second Amendment \| Delivered on April 1, 2016
Commitments And Contingencies [Line Items]
Monthly lease payments				$ 110,000
Square feet of leased property \| ft²				16,739
Operating lease payments commencement date				Sep. 01, 2016
Second Amendment \| Delivered on January 1, 2017
Commitments And Contingencies [Line Items]
Monthly lease payments				$ 23,000
Square feet of leased property \| ft²				3,384
Operating lease payments commencement date				May 01, 2017

Commitments and Contingencies - Schedule of Future Minimum Payments Required Under Leases (Details) $ in Thousands	Dec. 31, 2015 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Operating Lease, 2016	$ 2,522
Operating Lease, 2017	3,122
Operating Lease, 2018	3,122
Operating Lease, 2019	3,122
Operating Lease, 2020	3,122
Operating Lease, Thereafter	17,687
Operating Lease, Total	32,697
Capital Lease, 2016	8
Capital Lease, 2017	5
Capital Lease, 2018	1
Capital Lease	14
Capital Lease, Present value of minimum lease payments at December 31, 2015	14
Operating and Capital Lease, 2016	2,530
Operating and Capital Lease, 2017	3,127
Operating and Capital Lease, 2018	3,123
Operating and Capital Lease, 2019	3,122
Operating and Capital Lease, 2020	3,122
Operating and Capital Lease, Thereafter	17,687
Operating and Capital Lease, Total	$ 32,711

Net Loss per Share - Calculation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net loss	$ (19,857)	$ (19,475)	$ (10,689)	$ (10,695)	$ (10,370)	$ (9,348)	$ (7,618)	$ (9,697)	$ (60,716)	$ (37,034)	$ (13,167)
Accretion on preferred stock										(86,899)	(55,886)
Net loss applicable to common stockholders									$ (60,716)	$ (123,933)	$ (69,053)
Denominator:
Weighted-average number of common shares – basic and diluted									26,469,170	15,406,386	544,002
Net loss per share applicable to common stockholders—basic and diluted	$ (0.66)	$ (0.68)	$ (0.40)	$ (0.53)	$ (0.52)	$ (0.47)	$ (0.39)	$ (43.37)	$ (2.29)	$ (8.04)	$ (126.94)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net Loss per Share - Anti-Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share (Details) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	2,447,776	1,948,491
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	2,206,635	1,526,346
Restricted Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	216,716	422,145
Restricted Stock Units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from calculation of diluted net loss per share	24,425

Quarterly Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Operating expenses	$ 20,050	$ 19,678	$ 10,889	$ 10,896	$ 10,607	$ 9,584	$ 7,840	$ 9,909	$ 61,513	$ 37,940	$ 15,933
Loss from operations	(20,050)	(19,678)	(10,889)	(10,896)	(10,607)	(9,584)	(7,840)	(9,909)	(61,513)	(37,940)	(15,933)
Other income (expense), net	193	203	200	201	237	236	222	212
Net loss	$ (19,857)	$ (19,475)	$ (10,689)	$ (10,695)	$ (10,370)	$ (9,348)	$ (7,618)	$ (9,697)	$ (60,716)	$ (37,034)	$ (13,167)
Net loss per share applicable to common stockholders—basic and diluted	$ (0.66)	$ (0.68)	$ (0.40)	$ (0.53)	$ (0.52)	$ (0.47)	$ (0.39)	$ (43.37)	$ (2.29)	$ (8.04)	$ (126.94)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Subsequent Event - Additional Information (Details) - Development and Commercialize Collaboration Agreement - Subsequent Event - Upfront $ in Millions	1 Months Ended
	Jan. 31, 2016 USD ($)
Subsequent Event [Line Items]
Milestone revenue	$ 40.0
Milestone revenue refundable	$ 20.0

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical)	12 Months Ended
	Dec. 31, 2013 USD ($)
Series X Preferred Stock
Issuance cost	$ 42,096
Series C redeemable convertible preferred stock
Issuance cost	$ 1,151,923

Stockholders' Equity - Summary of Reserved Shares of Common Stock for Future Issuance (Parenthetical) (Details) - Subsequent Event - shares	Feb. 28, 2016	Jan. 01, 2016
2014 Plans
Class of Stock [Line Items]
Increase in common stock, shares reserved for future issuance		986,800
Employee Stock Purchase Plan
Class of Stock [Line Items]
Increase in common stock, shares reserved for future issuance	379,430