Ultragenyx Pharmaceutical (RARE) 10-K 27 Mar 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Mar. 23, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	RARE
Entity Registrant Name	Ultragenyx Pharmaceutical Inc.
Entity Central Index Key	1515673
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		35,596,219
Entity Public Float			$910.60

BALANCE_SHEETS

BALANCE SHEETS (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$24,324	$7,427
Short-term investments	163,163	45,950
Prepaid expenses and other current assets	5,929	1,848
Total current assets	193,416	55,225
Property and equipment, net	3,033	1,325
Restricted cash	744	451
Other assets	774	2,648
Total assets	197,967	59,649
Current liabilities:
Accounts payable	4,857	1,437
Accrued liabilities	7,575	4,406
Deferred rent—current portion	85	78
Total current liabilities	12,517	5,921
Convertible preferred stock warrant liability		3,419
Other liabilities	505	200
Total liabilities	13,022	9,540
Commitments and contingencies (Note 12)
Stockholders’ equity (deficit):
Preferred stock, par value of $0.001 per share—25,000,000 shares authorized; nil outstanding as of December 31, 2014 and 2013
Common stock, par value of $0.001 per share—250,000,000 shares authorized; 31,934,682 and 3,766,289 shares issued and outstanding as of December 31, 2014 and 2013	32	4
Additional paid-in capital	324,128
Accumulated other comprehensive income (loss)	-174	11
Accumulated deficit	-139,041	-74,836
Total stockholders’ equity (deficit)	184,945	-74,821
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	197,967	59,649
Series A Redeemable Convertible Preferred Stock
Current liabilities:
Convertible preferred stock		51,001
Series B Convertible Preferred Stock
Current liabilities:
Convertible preferred stock		$73,929

BALANCE_SHEETS_Parenthetical

BALANCE SHEETS (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Preferred stock, par value	$0.00	$0.00
Preferred stock, shares authorized	25,000,000	25,000,000
Preferred stock, shares outstanding	0	0
Common stock, par value	$0.00	$0.00
Common stock, shares authorized	250,000,000	250,000,000
Common stock, shares issued	31,934,682	31,934,682
Common stock, shares outstanding	3,766,289	3,766,289
Series A Redeemable Convertible Preferred Stock
Convertible preferred stock, par value	$0.00	$0.00
Convertible preferred stock, shares authorized	0	35,377,556
Convertible preferred stock, shares issued	0	34,349,894
Convertible preferred stock, shares outstanding	0	34,349,894
Series B Convertible Preferred Stock
Convertible preferred stock, par value	$0.00	$0.00
Convertible preferred stock, shares authorized	0	27,081,680
Convertible preferred stock, shares issued	0	27,081,680
Convertible preferred stock, shares outstanding	0	27,081,680

STATEMENTS_OF_OPERATIONS

STATEMENTS OF OPERATIONS (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating expenses:
Research and development	$45,967	$27,829	$12,641
General and administrative	10,811	4,451	3,344
Total operating expenses	56,778	32,280	15,985
Loss from operations	-56,778	-32,280	-15,985
Other income (expense), net:
Interest income	608	216	1
Other expense, net	-3,632	-3,006	-350
Total other income (expense), net	-3,024	-2,790	-349
Net loss	-59,802	-35,070	-16,334
Net loss attributable to common stockholders	($64,610)	($50,289)	($19,561)
Net loss per share attributable to common stockholders, basic and diluted	($2.25)	($14.87)	($14.20)
Shares used in computing net loss per share attributable to common stockholders, basic and diluted	28,755,758	3,382,489	1,377,207

STATEMENTS_OF_COMPREHENSIVE_LO

STATEMENTS OF COMPREHENSIVE LOSS (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net loss	($59,802)	($35,070)	($16,334)
Other comprehensive income (loss):
Unrealized gain (loss) on available-for-sale securities	-185	11
Total comprehensive loss	($59,987)	($35,059)	($16,334)

STATEMENTS_OF_CONVERTIBLE_PREF

STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND SHAREHOLDERS' EQUITY (DEFICIT) (USD $)	Total	Series A Redeemable Convertible Preferred Stock	Series B Convertible Preferred Stock	Employee Stock Option	Founders Stock	Common Stock	Additional Paid-in Capital	Additional Paid-in Capital	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income
In Thousands, except Share data								Employee Stock Option	Founders Stock
Beginning balance at Dec. 31, 2011	($7,961)					$3	$191			($8,155)
Beginning balance at Dec. 31, 2011		18,604
Beginning balance, shares at Dec. 31, 2011						3,038,620
Beginning balance, shares at Dec. 31, 2011		18,052,464
Issuance of common stock upon exercise of stock options and warrants	131						131
Issuance of common stock upon exercise of stock options and warrants , shares	420,308					420,308
Issuance of convertible preferred stock net of issuance costs		15,080	73,929
Issuance of convertible preferred stock net of issuance costs, shares		14,604,895	27,081,680
Issuance of Series A redeemable convertible preferred stock in lieu of cash dividend	-2,070	2,070					-1,205			-865
Issuance of Series A redeemable convertible preferred stock in lieu of cash dividend, shares		1,692,535
Employee stock-based compensation				178	713			178	713
Accretion on convertible preferred stock	-1,704	1,704								-1,704
Net loss	-16,334									-16,334
Ending balance at Dec. 31, 2012	-27,047					3	8			-27,058
Ending balance at Dec. 31, 2012		37,458	73,929
Ending balance, shares at Dec. 31, 2012						3,458,928
Ending balance, shares at Dec. 31, 2012		34,349,894	27,081,680
Issuance of common stock upon exercise of stock options and warrants	171					1	170
Issuance of common stock upon exercise of stock options and warrants , shares	307,366					307,361
Employee stock-based compensation				389	268			389	268
Accretion on convertible preferred stock	-13,543	13,543					-835			-12,708
Net loss	-35,070									-35,070
Unrealized gain (loss) on available-for-sale securities	11										11
Ending balance at Dec. 31, 2013	-74,821					4				-74,836	11
Ending balance at Dec. 31, 2013		51,001	73,929
Ending balance, shares at Dec. 31, 2013	3,766,289					3,766,289
Ending balance, shares at Dec. 31, 2013		34,349,894	27,081,680
Issuance of common stock upon exercise of stock options and warrants	736						736
Issuance of common stock upon exercise of stock options and warrants , shares	305,090					331,605
Employee stock-based compensation	5,394						5,394
Accretion on convertible preferred stock	-4,430	4,430					-27			-4,403
Net loss	-59,802									-59,802
Unrealized gain (loss) on available-for-sale securities	-185										-185
Conversion of convertible preferred stock to common stock at initial public offering	129,360	-55,431	-73,929			19	129,341
Conversion of convertible preferred stock to common stock at initial public offering, shares		-34,349,894	-27,081,680			19,598,486
Reclassification of preferred stock warrant liability	6,743						6,743
Issuance of common stock in connection with initial public offering, net of issuance costs and preferred stock dividend	121,711					7	121,704
Issuance of common stock in connection with initial public offering, net of issuance costs and preferred stock dividend, shares						6,624,423
Issuance of common stock in connection with underwritten public offering, net of issuance costs	60,239					2	60,237
Issuance of common stock in connection with underwritten public offering, net of issuance costs, shares						1,613,879
Ending balance at Dec. 31, 2014	$184,945					$32	$324,128			($139,041)	($174)
Ending balance, shares at Dec. 31, 2014	3,766,289					31,934,682
Ending balance, shares at Dec. 31, 2014		0	0

STATEMENTS_OF_CONVERTIBLE_PREF1

STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND SHAREHOLDERS' EQUITY (DEFICIT) (Parenthetical) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014
Series A | Redeemable Convertible Preferred Stock
Issuance of convertible preference stock, per share	$1.03
Issuance of convertible preference stock	$20
Series B | Convertible Preferred Stock
Issuance of convertible preference stock, per share	$2.77
Issuance of convertible preference stock	$1,071

STATEMENTS_OF_CASH_FLOWS

STATEMENTS OF CASH FLOWS (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating activities:
Net loss	($59,802)	($35,070)	($16,334)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	684	444	313
Amortization of premium on investment securities	3,600	1,413
Stock-based compensation	5,394	657	891
Revaluation of convertible preferred stock warrant liability	3,324	2,901	302
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	-4,081	-1,593	-47
Other assets	-411	-2,615	8
Accounts payable	3,177	237	858
Accrued expenses, deferred rent and other liabilities	3,481	2,426	1,505
Net cash used in operating activities	-44,634	-31,200	-12,504
Investing activities:
Purchase of property and equipment	-2,149	-407	-1,091
Purchase of investments	-208,972	-63,953
Proceeds from sale of investments	5,002
Proceeds from maturities of investments	82,972	16,601
Decrease (increase) in restricted cash	-293	25	-100
Net cash used in investing activities	-123,440	-47,734	-1,191
Financing activities:
Proceeds from issuance of convertible preferred stock, net			89,009
Proceeds from issuance of common stock, net	189,317	171	131
Payment of preferred stock dividend	-4,346
Proceeds from issuance of promissory notes			100
Net cash provided by financing activities	184,971	171	89,240
Net increase (decrease) in cash and cash equivalents	16,897	-78,763	75,545
Cash and cash equivalents at beginning of year	7,427	86,190	10,645
Cash and cash equivalents at end of year	24,324	7,427	86,190
Supplemental disclosures of non-cash investing and financing information:
Reclassification of warrant liability to equity upon conversion to common stock warrants	6,743
Series A Preferred Stock
Financing activities:
Proceeds from issuance of convertible preferred stock, net		33,623
Supplemental disclosures of non-cash investing and financing information:
Issuance of Series A redeemable convertible preferred stock in lieu of cash dividend			2,070
Series A and Series B
Supplemental disclosures of non-cash investing and financing information:
Conversion of Series A and Series B preferred stock to common stock	$129,360

Organization_and_Basis_of_Pres

Organization and Basis of Presentation	12 Months Ended
	Dec. 31, 2014
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization and Basis of Presentation	1.       Organization and Basis of Presentation
	Ultragenyx Pharmaceutical Inc. (the Company) is a biopharmaceutical company and was incorporated in California on April 22, 2010. The Company subsequently reincorporated in the state of Delaware in June 2011.
	The Company is focused on the identification, acquisition, development, and commercialization of novel products for the treatment of rare and ultra-rare diseases, with a focus on serious, debilitating metabolic genetic diseases. The Company is currently conducting a Phase 2 extension study of sialic acid, extended release (SA-ER) in patients with hereditary inclusion body myopathy (HIBM), a progressive muscle-wasting disorder; a Phase 3 study of recombinant human beta-glucuronidase (rhGUS) in patients with mucopolysaccharidosis 7, or MPS 7, a rare lysosomal storage disease; a Phase 2 clinical study for triheptanoin in patients with glucose transporter type-1 deficiency syndrome (Glut1 DS), a brain energy deficiency; a Phase 2 clinical study of triheptanoin in patients severely affected by long-chain fatty acid oxidation disorders (LC-FAOD), a genetic disorder in which the body is unable to convert long chain fatty acids into energy; and Phase 2 studies of KRN23, an antibody targeting fibroblast growth factor 23, or FGF23, in patients with X-linked hypophosphatemia (XLH) and tumor-induced osteomalacia (TIO), both rare genetic diseases that impair bone growth mineralization. The Company is in the clinical stage as of December 31, 2014, and since inception has been engaged in developing its product candidates, raising capital and recruiting personnel. The Company operates in one reportable segment in the United States of America.
	In the course of its research activities, the Company has sustained operating losses and expects such losses to continue over the next several years. The Company’s ultimate success depends on the outcome of its research and development activities. From April 22, 2010 (Inception) through December 31, 2014, the Company has incurred cumulative net losses of $119.4 million. Management expects to incur additional losses in the future to conduct product research and development and recognizes the need to raise additional capital to fully implement its business plan. Through December 31, 2014, the Company has relied primarily on the proceeds from equity offerings to finance its operations.
	On January 30, 2014, the Company’s registration statements on Form S-1 (File Nos. 333-192244 and 333-193675) relating to its initial public offering (IPO) of its common stock were declared effective by the Securities and Exchange Commission (SEC). The shares began trading on The NASDAQ Global Select Market on January 31, 2014. The public offering price of the shares sold in the offering was $21.00 per share. The IPO closed on February 5, 2014 and included 6,624,423 shares of common stock, which included 864,054 shares of common stock issued pursuant to the over-allotment option granted to the underwriters. The Company received total proceeds from the offering of $129.4 million, net of underwriting discounts and commissions of $9.7 million. After deducting offering expenses of approximately $3.3 million and a cash dividend of $4.3 million, which was paid to the preferred stockholders on the closing date, net proceeds were approximately $121.7 million. Upon the closing of the IPO, all shares of convertible preferred stock then outstanding converted into 19,598,486 shares of common stock and the Series A convertible preferred stock warrants were converted into warrants to purchase common stock.
	In July 2014, the Company completed an underwritten public offering in which the Company sold 1,613,879 shares of common stock, which included 302,602 shares of common stock purchased by the underwriters pursuant to an option granted to them in connection with the offering, at a public offering price of $40.00 per share. In addition, certain existing stockholders sold 706,072 shares of common stock in the underwritten public offering at the same per-share price. The total proceeds that the Company received from the offering were approximately $60.7 million, net of underwriting discounts and commissions of approximately $3.9 million. After deducting estimated offering expenses payable of approximately $0.4 million, net proceeds were $60.2 million.
	In February 2015, the Company completed an underwritten public offering in which the Company sold 3,450,000 shares of common stock, which included 450,000 shares of common stock purchased by the underwriters pursuant to an option granted to them in connection with the offering, at a public offering price of $54.00 per share. The total proceeds that the Company received from the offering were approximately $175.1 million, net of underwriting discounts and commissions of approximately $11.2 million. After deducting estimated offering expenses payable of approximately $0.6 million, net proceeds were $174.5 million.
	Management believes that the Company’s existing capital resources, including net proceeds received from the closing of the underwritten public offering in February 2015, will be sufficient to fund current operations into 2018. The Company intends to raise additional capital through the issuance of equity, borrowings, or strategic alliances with partner companies. However, if such financing is not available at adequate levels, the Company will need to reevaluate its operating plans.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2.       Summary of Significant Accounting Policies
	Use of Estimates
	The accompanying financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”). The preparation of the financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent liabilities and the reported amounts of expenses in the financial statements and the accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trial accruals, fair value of assets and liabilities, convertible preferred stock and related warrants, common stock, income taxes and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates.
	Cash and Cash Equivalents
	The Company considers all highly liquid investments with original maturities of three months or less from the date of purchase to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts and corporate bonds.
	Short-Term Investments
	All investments have been classified as “available-for-sale” and are carried at estimated fair value as determined based upon quoted market prices or pricing models for similar securities. Management determines the appropriate classification of its investments in debt securities at the time of purchase and reevaluates such designation as of each balance sheet date. Unrealized gains and losses are excluded from earnings and are reported as a component of comprehensive loss. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on available-for-sale securities are included in interest income and other expense, net, respectively. The cost of securities sold is based on the specific-identification method. Interest on marketable securities is included in interest income.
	Concentration of Credit Risk and Other Risks and Uncertainties
	Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents, and short-term investments. The Company’s cash, cash equivalents, and short-term investments are held by financial institutions that management believes are of high credit quality. The Company’s investment policy limits investments to fixed income securities denominated and payable in U.S. dollars such as U.S. government obligations, money market instruments and funds, corporate bonds, and asset-backed securities and places restrictions on maturities and concentrations by type and issuer. Such deposits may, at times, exceed federally insured limits. The Company has not experienced any losses on its deposits of cash and cash equivalents and its accounts are monitored by management to mitigate risk. The Company is exposed to credit risk in the event of default by the financial institutions holding its cash and cash equivalents and corporate bond issuers to the extent recorded in the balance sheets.
	Property and Equipment
	Property and equipment are stated at cost, less accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the respective assets. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss, if any, is reflected in operations.
	The useful lives of the property and equipment are as follows:
	Research and development equipment	5 years
	Furniture and office equipment	5 years
	Computer equipment	3 years
	Software	3 years
	Leasehold improvements	Shorter of lease term or estimated useful life
	Impairment of Long-Lived Assets
	The Company evaluates its long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows expected to result from the use of the asset and its eventual disposition. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset. The Company has not recorded impairment of any long-lived assets since inception.
	Restricted Cash
	Restricted cash includes a money market account with one of the Company’s financial institutions as collateral for its obligations under its facility lease of the Company’s corporate headquarters in Novato, California. Restricted cash also includes a savings account associated with a credit card agreement at one of the Company’s financial institutions.
	Deferred Offering Costs
	Deferred offering costs, which primarily consist of direct incremental legal and accounting fees relating to the IPO and the Company’s underwritten public offering in July 2014, were initially capitalized. IPO deferred offering costs of $3.3 million were subsequently offset against IPO proceeds upon the closing of the offering in February 2014. Underwritten public offering costs of $444,000 were subsequently offset against offering proceeds upon the completion of the underwritten public offering in July 2014. As of December 31, 2014 and 2013, deferred offering costs of $0 and $2.3 million, respectively, were capitalized and included in other assets on the balance sheet.
	Accruals of Research and Development Costs
	The Company records accruals for estimated costs of research, preclinical and clinical studies and manufacturing development. These costs are a significant component of the Company’s research and development expenses. A substantial portion of the Company’s ongoing research and development activities are conducted by third-party service providers, including contract research organizations. The Company accrues the costs incurred under its agreements with these third parties based on actual work completed in accordance with agreements established with these third parties. The Company determines the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of the services and the agreed-upon fee to be paid for such services. The Company makes significant judgments and estimates in determining the accrual balance in each reporting period. As actual costs become known, the Company adjusts its accruals. The Company has not experienced any material deviations between accrued clinical trial expenses and actual clinical trial expenses. However, actual services performed, number of patients enrolled, and the rate of patient enrollment may vary from the Company’s estimates, resulting in adjustments to clinical trial expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations.
	Leases
	The Company enters into lease agreements for its office and laboratory facilities. These leases are classified as operating leases. Rent expense is recognized on a straight-line basis over the term of the lease and, accordingly, the Company records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. Incentives granted under the Company’s facilities leases, including allowances to fund leasehold improvements, are deferred and are recognized as adjustments to rental expense on a straight-line basis over the term of the lease.
	Convertible Preferred Stock Warrant Liability
	Warrants for shares that are either puttable or that are contingently redeemable are classified as liabilities on the balance sheets. The warrants were subject to remeasurement at each balance sheet date, with changes in fair value recognized as a component of other expense, net. The Company continued to adjust the liability for changes in fair value until the completion of its IPO, whereby the warrants were converted to common stock warrants that are no longer subject to remeasurement.
	Convertible Preferred Stock
	The Company initially records all shares of convertible preferred stock net of offering costs on the dates of issuance, which represents the carrying value. Only the passage of time was required for Series A convertible preferred stock to become redeemable; accordingly, the Company was accreting the carrying value of Series A convertible preferred stock to its redemption value over the period from the date of issuance to June 16, 2017 (the earliest redemption date). The convertible preferred stock was classified outside of stockholders’ equity (deficit) on the accompanying balance sheets, as Series A convertible preferred stock could have been redeemed and as events triggering the liquidation preferences for both classes of convertible preferred stock were not solely within the Company’s control. In connection with the closing of Company’s IPO in February 2014, all of the Company’s outstanding convertible preferred stock was converted into common stock.
	Comprehensive Loss
	Comprehensive loss is the change in stockholders’ equity (deficit) from transactions and other events and circumstances other than those resulting from investments by stockholders and distributions to stockholders. The Company’s other comprehensive loss is comprised of unrealized gains and losses on investments in available-for-sale securities.
	Research and Development
	Research and development costs are expensed as incurred and consist of salaries and benefits, stock-based compensation expense, lab supplies and facility costs, as well as fees paid to other nonemployees and entities that conduct certain research and development activities on the Company’s behalf. Amounts incurred in connection with license agreements are also included in research and development expense. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed.
	Stock-Based Compensation
	Stock-based awards issued to employees, including stock options, are recorded at fair value as of the grant date using the Black-Scholes option-pricing model and recognized as expense on a straight-line basis over the employee’s requisite service period (generally the vesting period). Because noncash stock compensation expense is based on awards ultimately expected to vest, it is reduced by an estimate for future forfeitures. Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from estimates.
	Income Taxes
	The Company uses the liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. The Company must then assess the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance.
	The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.
	Net Loss per Share Attributable to Common Stockholders
	Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. The net loss attributable to common stockholders is calculated by adjusting the net loss of the Company for the accretion on the Series A convertible preferred stock and cumulative dividends on Series A and B convertible preferred stock. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since the effects of potentially dilutive securities are antidilutive.
	Reverse Stock Split
	In January 2014, the Company’s board of directors and its stockholders approved an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of shares of its common stock on a 1-for-3.1345 basis (the “Reverse Stock Split”). The par values and the authorized shares of the common and convertible preferred stock were not adjusted as a result of the Reverse Stock Split, nor were the outstanding shares of preferred stock. All issued and outstanding common stock and related per share amounts contained in the financial statements have been retroactively adjusted to reflect this Reverse Stock Split for all periods presented. A proportional adjustment to the conversion ratio for each series of convertible preferred stock was also effected in connection with the reverse split. The Reverse Stock Split was effected on January 17, 2014.
	Recent Accounting Pronouncements
	In June 2014, the FASB issued ASU 2014-10, Development Stage Enterprises (Topic 915): Elimination of Certain Financial Reporting Requirements, Including an Amendment to Variable Interest Entities Guidance in Topic 810, Consolidation (ASU 2014-10). ASU 2014-10 removes the definition of a development stage entity from the Master Glossary of the Accounting Standards Codification, thereby removing the financial reporting distinction between development stage entities and other reporting entities from U.S. GAAP. ASU 2014-10 also eliminates the requirements for development stage entities to (1) present inception-to-date information in the statements of income, cash flows, and shareholder equity, (2) label the financial statements as those of a development stage entity, (3) disclose a description of the development stage activities in which the entity is engaged, and (4) disclose in the first year in which the entity is no longer a development stage entity that in prior years it had been in the development stage. The amendments also clarify that the guidance in Topic 275, Risks and Uncertainties, is applicable to entities that have not commenced planned principal operations. The Company elected to early adopt the provisions of ASU 2014-10, as reflected in the financial statements.
	In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements — Going Concern (Subtopic 205-40): Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern, which requires management to evaluate, in connection with preparing financial statements for each annual and interim reporting period, whether there are conditions or events, considered in the aggregate, that raise substantial doubt about an entity’s ability to continue as a going concern within one year after the date that the financial statements are issued and provide related disclosures. This ASU will be effective for the Company in fiscal year 2016. Early adoption is permitted. We are currently assessing the future impact of this ASU in the financial statements.

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value Measurements	3.       Fair Value Measurements
	Financial assets and liabilities are recorded at fair value. The carrying amount of certain financial instruments, including cash and cash equivalents, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities. Assets and liabilities recorded at fair value on a recurring basis in the balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows:
	Level 1—Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date;
	Level 2—Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and
	Level 3—Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data.
	The Company’s financial instruments consist of Level 1 and Level 2 assets and Level 3 liabilities. Where quoted prices are available in an active market, securities are classified as Level 1. Money market funds are classified as Level 1.  Level 2 assets consist primarily of corporate bonds, asset backed securities and U.S. Government agency securities based upon quoted market prices for similar movements in active markets, quoted prices for identical or similar instruments in markets that are not active and model-based valuation techniques for which all significant inputs are observable in the market or can be corroborated by observable market data for substantially the full term of the assets. Where applicable these models project future cash flows and discount the future amounts to a present value using market-based observable inputs obtained from various third party data providers, including but not limited to, benchmark yields, interest rate curves, reported trades, broker/dealer quotes and reference data. In certain cases where there is limited activity or less transparency around inputs to valuation, securities are classified as Level 3. Level 3 liabilities consist of the convertible preferred stock warrant liability.
	The following table sets forth the fair value of the Company’s financial assets and liabilities remeasured on a recurring basis based on the three-tier fair value hierarchy (in thousands):
		31-Dec-14
		Level 1			Level 2		Level 3		Total
	Financial Assets:
	Money market funds	$	8,627		$	—	$	—	$	8,627
	Corporate bonds		—			152,942		—		152,942
	Asset backed securities		—			9,542		—		9,542
	U.S. Government agency securities		—			4,485		—		4,485
	Other		—			209		—		209
	Total financial assets	$	8,627		$	167,178	$	—	$	175,805
		31-Dec-13
		Level 1			Level 2		Level 3		Total
	Financial Assets:
	Money market funds	$	6,847		$	—	$	—	$	6,847
	Commercial paper		—			1,000		—		1,000
	Corporate bonds		—			44,950		—		44,950
	Total financial assets	$	6,847		$	45,950	$	—	$	52,797
	Financial Liabilities:
	Convertible preferred stock warrant liability	$	—		$	—	$	3,419	$	3,419
	Total financial liabilities	$	—		$	—	$	3,419	$	3,419
	The convertible preferred stock warrant liability was classified as a Level 3 liability. As of December 31, 2013, the Company determined the estimated fair value of the warrants using an option-pricing method to allocate the equity value of the Company to the warrants based on the Company’s capital structure. The equity value was estimated using the back-solve method, whereby the equity value was derived from a recent transaction involving the Company’s own securities. The key inputs used to determine value of the warrants was an estimated fair value of the Company’s common stock of $12.14 per share, expected volatility of 70%, the expected time to liquidity event of 0.43 years and risk-free interest rate of 0.11%. The significant unobservable input used in the fair value measurement of the convertible preferred stock warrant liability was the equity value of the Company. Generally, increases (decreases) in the equity value of the Company would result in a directionally similar impact to the fair value measurement of the preferred stock warrant liability.
	As of January 30, 2014, the Company determined the estimated fair value of the warrants using the Black-Scholes option-pricing model. Inputs used to determine the fair value included the value of the Company’s common stock upon closing of the IPO of $21.00, the remaining contractual term of the warrants of 7.0 years, risk-free interest rate of 2.19% and expected volatility of 70%. The preferred stock warrants were converted to common stock warrants upon the completion of the IPO and are no longer subject to remeasurement.
	The following table sets forth a summary of the changes in the estimated fair value of the Company’s convertible preferred stock warrants, which were measured at fair value on a recurring basis until their conversion to common stock warrants and related reclassification to additional paid-in capital (in thousands):
		Year Ended December 31,
		2014			2013
	Fair value, beginning of year	$	3,419		$	518
	Change in fair value recorded as a loss in other expense, net		3,324			2,901
	Reclassification of warrant liability to additional paid-in		(6,743	)		—
	capital
	Fair value, end of year	$	—		$	3,419
	The determination of the fair value of the convertible preferred stock warrants is discussed in Note 6. Generally, increases or decreases in the fair value of the underlying convertible preferred stock would result in a directionally similar impact in the fair value measurement of the warrant liability.

Balance_Sheet_Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2014
Investments Debt And Equity Securities [Abstract]
Balance Sheet Components	4.       Balance Sheet Components
	Cash Equivalents and Short-term Investments
	The fair values of cash equivalents, restricted cash and short-term investments classified as available-for-sale securities, consisted of the following (in thousands):
		31-Dec-14
						Gross Unrealized
		Amortized				Gains				Losses			Estimated Fair Value
		Cost
	Money market funds classified as cash equivalents	$	8,627			$	—			$	—		$	8,627
	Corporate bonds classified as cash equivalents		3,806				1				—			3,807
	Commercial Paper classified as short-term investments		—				—				—			—
	Corporate bonds classified as short-term investments		149,303				4				(172	)		149,135
	Asset backed securities		9,546				—				(4	)		9,542
	U.S. Government agency securities classified as short-term		4,488				1				(4	)		4,485
	investments
	Other		209				—				—			209
	Total	$	175,979			$	6			$	(180	)	$	175,805
		31-Dec-13
						Gross Unrealized
		Amortized Cost				Gains				Losses			Estimated Fair Value
	Money market funds classified as cash equivalents	$	6,847			$	—			$	—		$	6,847
	Commercial Paper classified as short-term investments		1,000				—				—			1,000
	Corporate bonds classified as short-term investments		44,939				17				(6	)		44,950
	Total	$	52,786			$	17			$	(6	)	$	52,797
	At December 31, 2014, the remaining contractual maturities of available-for-sale securities were less than two years. There have been no significant realized gains or losses on available-for-sale securities for the periods presented.
	Property and Equipment, net
	Property and equipment, net consists of the following (in thousands):
			December 31,
			2014				2013
	Research and development equipment		$	712			$	277
	Furniture and office equipment			572				385
	Computer equipment			268				248
	Software			821				58
	Leasehold improvements			2,141				1,154
	Property and equipment, gross			4,514				2,122
	Less accumulated depreciation			(1,481	)			(797	)
	Property and equipment, net		$	3,033			$	1,325
	Depreciation expense for the years ended December 31, 2014, 2013 and 2012 was $684,000, $444,000 and $313,000 respectively. Amortization of leasehold improvements and software is included in depreciation expense.
	Accrued Liabilities
	Accrued liabilities consist of the following (in thousands):
			December 31,
			2014				2013
	Research and clinical trial expenses		$	2,703			$	1,886
	Payroll and related expenses			4,205				2,360
	Other			667				160
	Total accrued liabilities		$	7,575			$	4,406

License_and_Research_Agreement

License and Research Agreements	12 Months Ended
	Dec. 31, 2014
Research Grant Agreement [Abstract]
License and Research Agreements	5.       License and Research Agreements
	Nobelpharma License Agreement
	In September 2010, the Company entered into a collaboration and license agreement with Nobelpharma Co., Ltd. (Nobelpharma). Under the terms of this collaboration and license agreement, each party granted the other party a worldwide exclusive license under certain of that party’s intellectual property related to the compound identified as N-acetylneuraminic acid, also known as sialic acid, to develop, manufacture, and commercialize products. Nobelpharma’s licensed territory includes Japan and certain other Asian countries, and the Company’s licensed territory includes the rest of the world.
	Under the collaboration and license agreement, the Company paid Nobelpharma $110,500 (10 million Yen) for the license, which was recorded as research and development expense in 2010, and also issued 76,567 shares of common stock to Nobelpharma with a minimal value. The Company is required to pay Nobelpharma annual royalties and earned royalties based on net sales upon product sales commencement. In addition, the Company is required to make certain payments to Nobelpharma based upon achievement of certain development and approval milestones. The remaining total aggregate payments, if all milestones are achieved by Nobelpharma, would be 200 million Yen (approximately $1.7 million based on the exchange rate at December 31, 2014). The Company will pay a high single digit royalty on net sales in the Company’s territory and will receive a mid-single digit royalty on net sales in the Nobelpharma territory, excluding Japan, if such product sales are ever achieved. Net sales, as defined in the collaboration and license agreement, represent the net sales of products whereby the licensed compound is the active ingredient. If the products include other active ingredients, the portion of the net sales allocated to the licensed compound would be used in determining the royalty payments.
	Saint Louis University License Agreement
	In November 2010, the Company entered into a license agreement with Saint Louis University (SLU). Under the terms of this license agreement, SLU granted the Company an exclusive worldwide license to make, have made, use, import, offer for sale, and sell therapeutics related to SLU’s beta-glucuronidase product for use in the treatment of human diseases.
	Under the license agreement, the Company paid SLU an up-front fee of $10,000, which was recorded as research and development expense in 2010. The Company will be required to make a milestone payment of $100,000 upon approval of a glucuronidase-based enzyme therapy for treatment of MPS 7. Additionally, upon reaching a certain level of cumulative worldwide sales of the product, the Company will be required to pay to SLU a low single-digit royalty on net sales of the licensed products in any country or region, if such product sales are ever achieved.
	AAIPharma License Agreement
	In March 2011, the Company entered into a license agreement with AAIPharma Services Corp. (AAIPharma). Under the terms of this license agreement, AAI Pharma granted the Company a fully paid-up, royalty-free, exclusive, perpetual, and irrevocable license to research, develop, make, have made, use, import, offer for sale, and sell products incorporating AAIPharma’s controlled release matrix solid dose oral tablet. Under the license agreement, the Company will pay a mid-single digit percentage of any sublicense revenue received by Ultragenyx related to the sublicense of AAIPharma technology that had been initially licensed by Ultragenyx.
	HIBM Research Group
	License Agreement
	In April 2012, the Company entered into an exclusive license agreement with HIBM Research Group (HRG). Under the terms of this license agreement, HRG granted the Company an exclusive worldwide license to certain intellectual property related to the treatment of HIBM. Under the license agreement, the Company paid HRG an up-front fee of $25,000 which was recorded as research and development expense during the year ended December 31, 2012. The Company may make future payments that aggregate up to $300,000 and that are contingent upon attainment of various development and approval milestones. Additionally, the Company will pay to HRG a royalty of less than 1% of net sales of the licensed products in the licensed territories, if such product sales are ever achieved.
	St. Jude Children’s Research Hospital License Agreement
	In September 2012, the Company entered into a license agreement with St. Jude Children’s Research Hospital (St. Jude). Under the terms of this license agreement, St. Jude granted the Company an exclusive license under certain know-how to research, develop, make, use, offer to sell, import, and otherwise commercialize and exploit St. Jude’s protective protein, cathepsin, a protein product to treat, prevent, and/or diagnose galactosialidosis and other monogenetic diseases.
	Under the license agreement, the Company paid St. Jude an up-front fee of $10,000 which was recorded as research and development expense during the year ended December 31, 2012. Additionally, the Company will pay to St. Jude a royalty of less than 1% on net sales of the licensed products in the licensed territories, if such product sales are ever achieved.
	Baylor Research Institute License Agreement
	In September 2012, the Company entered into a license agreement with Baylor Research Institute (BRI). Under the terms of this license agreement, BRI exclusively licensed to the Company certain intellectual property related to triheptanoin for North America. Under the license agreement, the Company paid BRI an up-front fee of $250,000 which was recorded as research and development expense during the year ended December 31, 2012. In June 2013, the Company notified BRI that it was exercising its option pursuant to the agreement to license the rights to triheptanoin in all territories outside of the United States, Canada and Mexico and paid the option exercise fee of $750,000 which was recorded as research and development expense.
	The Company may make future payments of up to $10.5 million contingent upon attainment of various development milestones and $7.5 million contingent upon attainment of various sales milestones. Additionally, the Company will pay to BRI a mid-single digit royalty on net sales of the licensed product in the licensed territories, if such product sales are ever achieved.
	Kyowa Hakko Kirin Collaboration and License Agreement
	In August 2013, the Company entered into a collaboration and license agreement with Kyowa Hakko Kirin Co. Ltd. (KHK). Under the terms of this collaboration and license agreement, the Company and KHK will collaborate on the development and commercialization of certain products containing KRN23, an antibody directed towards FGF23, in the field of orphan diseases in the United States and Canada, or the profit share territory, and in the EU, Switzerland, and Turkey, or the European territory, and the Company will have the right to develop and commercialize such products in the field of orphan diseases in Mexico and Central and South America, or Latin America. In the field of orphan diseases, and except for ongoing studies being conducted by KHK, the Company will be the lead party for development activities in the profit share territory and in the European territory until the applicable transition date. The Company will share the costs for development activities in the profit share territory and European territory conducted pursuant to the development plan before the applicable transition date equally with KHK. On the applicable transition date in the relevant territory, KHK will become the lead party and be responsible for these costs. However, the Company will continue to share the costs of the studies commenced prior to the applicable transition date equally with KHK. The Company has the primary responsibility for conducting certain research and development services. The Company is obligated to provide assistance in accordance with the agreed upon development plan as well as participate on various committees. If KRN23 is approved, the Company and KHK will share commercial responsibilities and profits in the profit share territory until the applicable transition date, KHK will commercialize KRN23 in the European territory and the Company will develop and commercialize KRN23 in Latin America. KHK will manufacture and supply KRN23 for clinical use globally and will manufacture and supply KRN23 for commercial use in the profit share territory and Latin America.
	The Company is accounting for the agreement as a collaboration arrangement as defined in ASC 808, Collaborative Agreements; accordingly, the Company recorded $4.6 million and $402,000 for its share of the costs as research and development expenses for the years ended December 31, 2014 and 2013, respectively. As of December 31, 2014 and 2013, the Company had a receivable in the amount of $1.3 million and $370,000 from KHK and an accrued liability of $35,000 and $435,000, respectively, for this collaboration arrangement.

Convertible_Preferred_Stock_Wa

Convertible Preferred Stock Warrants and Common Stock Warrants	12 Months Ended
	Dec. 31, 2014
Warrants And Rights Note Disclosure [Abstract]
Convertible Preferred Stock Warrants and Common Stock Warrants	6.       Convertible Preferred Stock Warrants and Common Stock Warrants
	In connection with the terms of various promissory notes issued by the Company from June 2010 through February 2011, the Company issued warrants in which the number of shares and exercise price were subject to the per share price offered in the Series A convertible preferred stock sale. In February 2011, the Company entered into a convertible note and warrant purchase agreement with William Aliski, a member of the Company’s board of directors, in which it borrowed $250,000 from Mr. Aliski. In February 2011, the Company also entered into a convertible note and warrant purchase agreement with John Klock, a former member and former observer of the Company’s board of directors, in which it borrowed $1.5 million from Mr. Klock. In June 2011, in connection with its closing of the first round of Series A convertible preferred stock financing, all of the convertible notes were converted into Series A convertible preferred stock, and the Company determined that the warrants were convertible to 1,027,662 shares of Series A convertible preferred stock at an exercise price of $1.034 per share. The warrants are subject to certain anti-dilution provisions, including Series A dividends paid in shares of preferred stock. The Company determined the fair value of the warrants using an option-pricing method to allocate the equity value of the Company to the warrants based on the Company’s capital structure. The equity value was estimated using the back-solve method, whereby the equity value was derived from a recent transaction involving the Company’s own securities. The fair value ascribed to these warrants upon their issuance was $203,000. The fair value of the warrants was recorded as debt issuance costs and was amortized to interest expense using the effective-interest-rate method over the loan term. In connection with the conversion of the promissory notes into shares of Series A convertible preferred stock in June 2011, the Company recognized all remaining unamortized debt issuance costs. Upon the closing of the Company’s IPO in February 2014, the warrants were converted into warrants to purchase common stock. Accordingly, the warrants were reclassified from a liability to permanent equity and were no longer subject to remeasurement.
	As of December 31, 2014, outstanding warrants consisted of the following:
	Common Stock Warrants:	Number of Warrants		Date Issued	Term	Exercise
						Price
	Common stock		83,167	Jun-10	10 years	$	3.006
	Common stock		174,651	Feb-11	10 years		3.006
	Common stock		66,533	Jun-11	10 years		3.006
	Total common stock warrants		324,351
	As of December 31, 2013, outstanding warrants consisted of the following:
	Convertible Preferred Stock Warrants:	Number of Warrants		Date Issued	Term	Exercise
						Price
	Series A		241,803	Jun-10	10 years	$	1.034
	Series A		592,417	Feb-11	10 years		1.034
	Series A		193,442	Jun-11	10 years		1.034
	Total convertible preferred stock warrants		1,027,662
	The fair value of the warrants was estimated to be $6.7 million and $3.4 million as of January 30, 2014 (pricing date of IPO) and December 31, 2013, respectively. The Company recorded $3.3 million, $2.9 million and $302,000 to other expense, net, for the years ended December 31, 2014, 2013 and 2012 respectively, representing the change in fair value of the warrants for the respective period.

Common_Stock

Common Stock	12 Months Ended
	Dec. 31, 2014
Common Stock [Abstract]
Common Stock	7.     Common Stock
	The Company has reserved sufficient shares of common stock for issuance upon the exercise of stock options and the exercise of warrants. Common stockholders are entitled to dividends if and when declared by the board of directors, subject to the prior rights of the preferred stockholders. As of December 31, 2014, no common stock dividends had been declared by the board of directors.
	The Company had reserved shares of common stock, on an as-converted basis, for future issuance as follows:
		December 31,
		2014		2013
	Convertible preferred stock		—		19,598,486
	Convertible preferred stock warrants		—		353,459
	Common stock warrants		324,351		—
	Employee stock purchase plan		600,000		—
	2011 equity incentive plan		1,960,225		2,228,883
	2014 equity incentive plan		810,250		—
	Shares available for future stock option grants		1,439,750		799,963
			5,134,576		22,980,791

Convertible_Preferred_Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2014
Preferred Stock [Abstract]
Convertible Preferred Stock	8.     Convertible Preferred Stock
	Under the Company’s Amended and Restated Certificate of Incorporation, as amended, the Company is authorized to issue two classes of shares: preferred and common stock. The preferred stock is issuable in series, and the Company’s board of directors is authorized to determine the rights, preferences and terms of each series. As of December 31, 2014, no shares of preferred stock were outstanding. As of December 31, 2013, convertible preferred stock consisted of the following (in thousands, except share and per share amounts):
		As of December 31, 2013
	Convertible Preferred Stock:	Shares Authorized		Shares Issued and Outstanding		Proceeds Net of Issuance Costs		Aggregate Liquidation Preference
	Series A		35,377,556		34,349,894	$	33,623	$	38,063
	Series B		27,081,680		27,081,680		73,929		76,739
	Total convertible preferred stock		62,459,236		61,431,574	$	107,552	$	114,802
	The liquidation preference consists of the liquidation preference on the outstanding shares of the convertible preferred stock and the dividends in arrears on such shares in the amount of $4.0 million as of December 31, 2013.
	Significant provisions of the convertible preferred stock were as follows:
	Dividends —When and as declared by the Company’s board of directors, the holders of the Series A convertible preferred stock (Series A) and the Series B convertible preferred stock (Series B), collectively referred to as “Preferred Stock,” are entitled to receive dividends, out of any assets legally available therefor, prior and in preference to the declaration or payment of any dividend on the common stock or other securities and rights convertible of the Company, at the rate of $0.062 per share per annum, payable in the form of cash or property or, upon conversion of the preferred stock to common stock, payable in cash. During 2012, $2,070,000 of preferred stock dividends were declared and paid to holders of Series A in the form of additional shares of Series A. Dividends in arrears as of December 31, 2013 were $4.0 million for both series of preferred stock. In connection with the closing of the IPO in February 2014, all dividends in arrears in the amount of $4.3 million were paid to both series of preferred stock.
	Liquidation—In the event of any liquidation, dissolution, or winding up of the Company, either voluntary or involuntary, the holders of Series A and Series B shall be entitled to receive, on a pari passu basis, prior and in preference to any distribution of any of the assets of this Corporation to the holders of common stock by reason of their ownership thereof, an amount per share equal to the sum of $1.034 (the Original Series A Issue Price) for each outstanding share of Series A (subject to adjustment for recapitalizations) and $2.769 (the Original Series B Issue Price) for each outstanding share of Series B (subject to adjustment for recapitalizations) and an amount equal to all declared or accrued but unpaid dividends on such shares.
	Redemption—Only the passage of time was required for Series A to become redeemable; accordingly, the Company was accreting the carrying value of Series A to its redemption value over the period from the date of issuance to June 16, 2017 (the earliest redemption date). The Series B was not redeemable. Conversion —Each share of preferred stock, at the option of the holder, was convertible into common stock determined by dividing the Original Series A issue price or Original Series B issue price, as applicable, by the conversion price applicable to such share in effect on the date the certificate is surrendered for conversion, subject to certain provisions for adjustment of the conversion price. In connection with the reverse split which was effective on January 17, 2014, all shares of preferred stock were convertible into common stock on a 3.1345-for-one basis. Upon the closing of the IPO, all shares of the preferred stock then outstanding converted into 19,598,486 shares of common stock.

StockBased_Awards

Stock-Based Awards	12 Months Ended
	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Awards	9.     Stock-Based Awards
	2011 Equity Incentive Plan
	In 2011, the Company adopted the 2011 Equity Incentive Plan (the 2011 Plan). The 2011 Plan provides for the granting of stock-based awards to employees, directors, and consultants under terms and provisions established by the board of directors. Under the terms of the 2011 Plan, options may be granted at an exercise price not less than fair market value. For employees holding more than 10% of the voting rights of all classes of stock, the exercise prices for incentive stock options must be at least 110% of fair market of the common stock on the grant date, as determined by the board of directors. The terms of options granted under the 2011 Plan may not exceed ten years. Options granted generally vest over a period of four years. Typically, the vesting schedule for option grants to newly hired employees provides that 1/4 of the grant vests upon the first anniversary of the employee’s date of hire, with the remainder of the shares vesting monthly thereafter at a rate of 1/48 of the total shares subject to the option. All other employee options typically vest in equal monthly installments over the four-year vesting schedule. In connection with the Company’s IPO, no further grants will be made under this plan and all remaining shares available for grant were transferred to the 2014 Incentive Plan.
	2014 Incentive Plan
	In 2014, the Company adopted the 2014 Incentive Plan (the 2014 Plan), which became effective upon the closing of the Company’s IPO in February 2014. The 2014 Plan had 2,250,000 shares of common stock available for future issuance at the time of its inception, which included 655,038 shares available under the 2011 Plan, which were transferred to the 2014 Plan upon adoption. The 2014 Plan provides for automatic annual increases in shares available for grant, beginning on January 1, 2015 through January 1, 2024. The 2014 Plan provides for the granting of stock-based awards to employees, directors, and consultants under similar terms, conditions and provisions as the 2011 Plan.
	Stock Option Activity
	A summary of option activity under the 2011 Plan and the 2014 Plan and related information are as follows:
			Options Outstanding
			Number of Options				Weighted- Average Exercise Price				Weighted-Average Remaining Contractual Term (Years)				Aggregate Intrinsic Value
															(In thousands)
	Outstanding — December 31, 2011			1,451,580			$	0.31				9.88			$	—
	Shares reserved			—				—
	Options granted			499,278				0.81
	Options exercised			(420,308	)			0.31
	Options cancelled			(9,583	)			0.31
	Outstanding — December 31, 2012			1,520,967				0.47				9.11			$	2,038
	Options granted			1,267,797				5.8
	Options exercised			(307,366	)			0.55
	Options cancelled			(252,515	)			1.26
	Outstanding — December 31, 2013			2,228,883				3.41			8.91				$	19,468
	Shares reserved			—				—
	Options granted			932,555				41.12
	Options exercised			(305,090	)			2.14
	Options cancelled			(116,873	)			8.92
	Outstanding — December 31, 2014			2,739,475				16.15				8.43			$	79,840
	Vested and exercisable — December 31, 2014			802,189				2.65			7.55				$	33,072
	Vested and expected to vest — December 31,			2,666,378			$	15.86				8.41			$	78,390
	2014
	The aggregate intrinsic values of options outstanding, vested and exercisable, and vested and expected to vest were calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock as of December 31, 2014. The total intrinsic value of options exercised during the years ended December 31, 2014, 2013 and 2012 was $ 12.1 million, $1.0 million and $211,000, respectively.
	The options outstanding and exercisable by exercise price as of December 31, 2014 are as follows:
					Options Outstanding								Options Exercisable
	Exercise Price				Numbers Outstanding				Weighted-Average Remaining Contractual Term (in Years)				Number Exercisable				Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (in Years)
	$	0.31				667,554				6.88				472,481			$	0.31	6.88
	$0.81 to $1.82					453,552				7.98				136,284				1.33	7.98
	$4.07 to $6.86					523,169				8.81				125,181				6.59	8.81
	$11.19 to $21.00					315,950				9.01				68,243				14.26	9.01
	$33.55 to $34.93					249,500				9.4				—				—	—
	$40.37 to $49.15					203,750				9.8				—				—	—
	$53.03 to $58.02					326,000				9.43				—				—	—
						2,739,475				8.43				802,189			$	2.65	7.55
	The weighted-average estimated fair value of stock options granted was $26.58, $3.82 and $0.47 per share of the Company’s common stock during the years ended December 31, 2014, 2013, and 2012, respectively.
	The total estimated fair value of options vested during the years ended December 31, 2014, 2013 and 2012 was $2.0 million, $127,000 and $172,000, respectively.
	Restricted Stock Units
	In 2014, the Company granted 31,000 Restricted Stock Units (RSUs) under the 2014 Plan to employees with a weighted-average grant date fair value of $53.23. The fair value of the RSUs is determined on the grant date based on the fair value of the Company’s common stock. The fair value of the RSUs is recognized as expense ratably over the vesting period of two to four years. The total fair value of shares vested was $264,000 during 2014. As of December 31, 2014, the the total unrecognized compensation expense related to unvested RSUs, net of estimated forfeitures, was $1.4 million, which the Company expects to recognize over an estimated weighted-average period of 2.0 years
	Employee Stock Purchase Plan
	In January 2014, the Company adopted the 2014 Employee Stock Purchase Plan (the ESPP) which became effective upon the closing of our IPO in February 2014. The Company reserved a total of 600,000 shares of common stock for issuance under the ESPP. Eligible employees may purchase common stock at 85% of the lesser of the fair market value of common stock on the first or last day of the purchase period. The ESPP provides for automatic annual increases in shares available for grant, beginning on January 1, 2015 through January 1, 2024. The Company has not determined the date on which the initial purchase period will commence under the ESPP.
	Founder’s Stock
	In connection with the Series A preferred stock financing, the Company entered into a stock repurchase agreement with the founder on June 16, 2011, whereby 2,552,241 shares of common stock previously owned by the founder were subject to repurchase by the Company at the original issuance price in the event that the founder’s employment is terminated either voluntarily or involuntarily. The repurchase rights lapsed over a period of two years from June 16, 2011. The Company calculated the estimated fair value of these restricted shares at the time the restriction was added to the shares as $1,199,000 and recorded this amount as stock-based compensation ratably over the period that the repurchase rights lapsed. Stock-based compensation expense pertaining to the founder’s stock was $0, $268,000 and $713,000 during the years ended December 31, 2014, 2013, and 2012, respectively.
	Stock-Based Compensation Expense
	Total stock-based compensation recognized was as follows (in thousands):
			Year Ended December 31,
			2014				2013				2012
	Research and development		$	4,116			$	293			$	130
	General and administrative			1,278				364				761
	Total stock-based compensation expense		$	5,394			$	657			$	891
	As of December 31, 2014, the total unrecognized compensation expense related to unvested options, net of estimated forfeitures, was $22.5 million, which the Company expects to recognize over an estimated weighted-average period of 3.2 years.
	In determining the estimated fair value of the stock-based awards, the Company uses the Black-Scholes option-pricing model and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment to determine.
	Expected Term—The Company’s expected term represents the period that the Company’s stock-based awards are expected to be outstanding and is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term).
	Expected Volatility—Prior to the IPO in January 2014, the Company was privately held and did not have any trading history for its common stock; accordingly, the expected volatility was estimated based on the average volatility for comparable publicly traded biopharmaceutical companies over a period equal to the expected term of the stock option grants. When selecting comparable publicly traded biopharmaceutical companies on which it has based its expected stock price volatility, the Company selected companies with comparable characteristics to it, including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected life of the stock-based awards. The historical volatility data was computed using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. In 2014, the Company modified its approach by phasing in our own common stock trading history and supplemented the remaining historical information with a blended volatility from the trading history from the common stock of the same set of comparable publicly traded biopharmaceutical companies. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. Sufficient trading history does not yet exist for the Company’s common stock, therefore the estimate of expected volatility is based on the volatility of other companies with similar products under development, market, size and other factors.
	Risk-Free Interest Rate—The risk-free interest rate is based on the U.S. Treasury zero coupon issues in effect at the time of grant for periods corresponding with the expected term of option.
	Expected Dividend—The Company has never paid dividends on its common stock and has no plans to pay dividends on its common stock. Therefore, the Company used an expected dividend yield of zero.
	The fair value of stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions:
			Year Ended December 31,
			2014				2013				2012
Expected term		6.23 years				6.25 years				6.25 years
Expected volatility			70%				74%				67%
Risk-free interest rate			1.90%			0.7% to 1.6%					0.60%
Expected dividend rate			0.00%				0.00%				0.00%

Defined_Contribution_Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2014
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Defined Contribution Plan	10.      Defined Contribution Plan
	In March 2013, the Company began to sponsor a 401(k) retirement plan, in which substantially all of its full-time employees are eligible to participate. Eligible participants may contribute a percentage of their annual compensation to this plan, subject to statutory limitations. The Company did not provide any contributions during the year ended December 31, 2014.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	11.      Income Taxes
	The Company did not record a provision or benefit for income taxes during the years ended December 31, 2014, 2013 and 2012. The Company has incurred net operating losses since inception. The Company has not reflected any benefit of such net operating loss carryforwards in the accompanying financial statements. The Company has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding the realization of such assets.
	The effective tax rate of our provision for income taxes differs from the federal statutory rate as follows:
		Year Ended December 31,
		2014				2013				2012
	Federal statutory income tax rate		34		%		34		%		34		%
	State income taxes, net of federal benefit		4.6				5				6
	Federal tax credits		6.7				8.2				8.6
	Nondeductible permanent items		(1.2	)			(2.9	)			—
	Stock compensation		(1.4	)			(0.6	)			(1.9	)
	Change in valuation allowance		(42.7	)			(44.6	)			(46.0	)
	Uncertain tax positions		—				0.9				(0.7	)
	Provision for income taxes		—				—				—
	The tax effect of temporary differences that give rise to significant portions of the deferred tax assets is presented below (in thousands):
		Year Ended December 31,
		2014				2013
	Deferred tax assets:
	Net operating loss carryforwards	$	32,500			$	16,352
	Tax credits		16,737				8,692
	Stock options		1,058				—
	Accruals and reserves		476				591
	Fixed assets and intangibles		455				65
	Other		113				—
	Total deferred tax assets		51,339				25,700
	Valuation allowance		(51,339	)			(25,700	)
	Net deferred tax assets	$	—			$	—
	Additionally, the future utilization of the net operating loss carryforwards and tax credits to offset future taxable income may be subject to an annual limitation, pursuant to Internal Revenue Code Section 382, as a result of ownership changes that may have occurred previously or that could occur in the future. A Section 382 analysis to determine the limitation of the net operating loss carryforwards has not been performed. Until this analysis has been performed, the deferred tax assets for net operating losses of $725,000 generated through December 31, 2014 have been removed from the deferred tax asset schedule and a corresponding decrease to the valuation allowance has been recorded. This represents the amount estimated to expire before utilization, assuming a change in ownership has occurred. The Company recorded unrecognized tax benefits for uncertainty in income taxes. Due to the existence of the valuation allowance, future changes in unrecognized tax benefits will not impact the effective tax rate. The valuation allowance increased by $25.6 million and $15.6 million during the year ended December 31, 2014 and 2013, respectively.
	As of December 31, 2014 and 2013, the Company had approximately $89.0 million and $39.4 million of federal net operating loss carryforwards available to reduce future taxable income that will begin to expire in 2030 for federal tax purposes. As of December 31, 2014 and 2013, the Company had approximately $90.1 million and $50.8 million of state NOL carryforwards available to reduce future taxable income that will begin to expire in 2030 for state tax purposes.
	As of December 31, 2014 and 2013, the Company also had research and development tax credit carryforwards of approximately $986,000 and $464,000 for federal purposes and $1.6 million and $586,000 for state purposes, respectively, available to offset future taxable income. If not utilized, the federal carryforwards will expire in various amounts beginning in 2030, and the state credits can be carried forward indefinitely.
	On January 2, 2013, the American Taxpayer Relief Act of 2012 (the Act) was passed in to law. The Act included a retroactive extension of the U.S. research credit for 2012. Since the effects of the tax law changes are recognized in the first period the Company recognized $12,000 of additional research credits as a discrete item during 2013. The tax effects of the research credits will be offset by a valuation allowance and will not impact the financial statements.
	As of December 31, 2014 and 2013, the Company had an Orphan Drug Credit of approximately $21.9 million and $11.6 million for federal tax purposes available to offset future regular and alternative minimum tax. If not utilized, the Orphan Drug Credit will expire in various amounts beginning in 2026.
	A reconciliation of the Company’s unrecognized tax benefits for the years ended December 31, 2014 and 2013 is as follows (in thousands):
		December 31,
		2014				2013				2012
	Balance at beginning of year	$	3,725			$	1,551			$	324
	Additions based on tax positions related to current year		3,550				2,169				1,227
	Additions (reductions) for tax positions of prior years		—				5				—
	Balance at end of year	$	7,275			$	3,725			$	1,551
	The entire amount of the unrecognized tax benefits would not impact the Company’s effective tax rate if recognized. The Company has elected to include interest and penalties as a component of tax expense. During the years ended December 31, 2014 and 2013, the Company did not recognize accrued interest and penalties related to unrecognized tax benefits. The Company does not anticipate that the amount of existing unrecognized tax benefits will significantly increase or decrease during the next 12 months.
	The Company files income tax returns in the U.S. federal and California tax jurisdictions. The federal and state income tax returns from inception to December 31, 2014 remain subject to examination.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	12.       Commitments and Contingencies
	Facilities
	In July 2011, the Company entered into a lease agreement for office facilities in Novato, California, which provided for a tenant improvement allowance of up to $376,000. The lease commenced in March of 2012. This noncancelable operating lease expires five years after the commencement date. At the end of the lease term, the Company has the option to extend the lease for two additional consecutive terms of five years each. In July 2011, the Company signed an addendum to the lease agreement to add warehouse space to the arrangement. As provided in the lease agreement, monthly lease payments are subject to an annual increase based upon a CPI adjustment with a minimum increase of three percent and a maximum increase of six percent per year. In February 2014, the Company signed an addendum to the lease agreement to lease the remainder of the office and warehouse space of the building and extend the term of the leased space for an additional five years from the date of the addendum, as provided under the original agreement. The addendum provided for a tenant improvement allowance of up to $369,000.
	In September 2010, the Company entered into a license and service agreement for a lab facility in Novato, California. The term commenced in September 2010. In September 2012 and August 2014, the Company entered into amendments to increase the size of the lab facility and extend the term. The term of the lease under the September 2014 amendment expires five years after the amendment date and can be extended for three one-year periods after the amendment term, and either party may terminate the agreement with one year’s prior notice without cause.
	As of December 31, 2014, the aggregate future minimum lease payments under the noncancelable operating lease arrangements are as follows (in thousands):
	Year Ending December 31,
	2015	$	1,173
	2016		1,305
	2017		1,335
	2018		1,366
	2019		560
	Thereafter		—
		$	5,739
	*- Includes additional lease payments under the lease amendment entered into in March 2015, as detailed in Note 14 “Subsequent Events”
	The Company recognizes rent expense on a straight-line basis over the noncancelable term of its operating lease. Rent expense was $572,000, $285,000 and $265,000 during the years ended December 31, 2014, 2013 and 2012, respectively.
	Under the terms of the lease agreement and the addendum of its Novato office facility, the Company provided the lessor with a irrevocable letter of credit. The lessor shall be entitled to draw on the letter of credit in the event of any uncured default by the Company under the terms of the lease. Provided there has been no default on the lease, the Company may reduce the amount of the letter of credit by $119,000 on each anniversary date effective May 1, 2014. As of December 31, 2014, the current amount of restricted cash and amount of the irrevocable letter of credit in connection with the lease agreement was $594,000.
	Other Commitments
	The Company has various manufacturing, clinical, research, and other contracts with vendors in the conduct of the normal course of its business. All contracts are terminable, with varying provisions regarding termination. If a contract with a specific vendor were to be terminated, the Company would only be obligated for the products or services that the Company had received at the time the termination became effective.
	Contingencies
	While there are no legal proceedings the Company is aware of, the Company may become party to various claims and complaints arising in the ordinary course of business. Management does not believe that any ultimate liability resulting from any of these claims will have a material adverse effect on its results of operations, financial position, or liquidity. However, management cannot give any assurance regarding the ultimate outcome of these claims, and their resolution could be material to operating results for any particular period, depending upon the level of income for the period.
	Guarantees and Indemnifications
	The Company indemnifies each of its directors and officers for certain events or occurrences, subject to certain limits, while the director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and bylaws. The term of the indemnification period lasts as long as a director may be subject to any proceeding arising out of acts or omissions of such director in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director liability insurance. This insurance allows the transfer of risk associated with the Company’s exposure and may enable it to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, it has not recognized any liabilities relating to these obligations for any period presented.

Net_Loss_per_Share_Attributabl

Net Loss per Share Attributable to Common Stockholders	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Net Loss per Share Attributable to Common Stockholders	13.      Net Loss per Share Attributable to Common Stockholders
	The following table sets forth the computation of the basic and diluted net loss per share attributable to common stockholders during the years ended December 31, 2014, 2013 and 2012 (in thousands, except share and per share data):
		Year Ended December 31,
		2014			2013			2012
	Numerator:
	Net loss	$	(59,802	)	$	(35,070	)	$	(16,334	)
	Accretion and dividends on convertible preferred stock		(4,808	)		(15,219	)		(3,227	)
	Net loss attributable to common stockholders	$	(64,610	)	$	(50,289	)	$	(19,561	)
	Denominator:
	Weighted-average common shares outstanding		28,755,758			3,583,522			3,118,798
	Less: weighted-average unvested common shares subject to		—			(201,033	)		(1,741,591	)
	repurchase
	Weighted-average shares used to compute net loss per		28,755,758			3,382,489			1,377,207
	share attributable to common stockholders, basic and diluted
	Net loss per share attributable to common stockholders,	$	(2.25	)	$	(14.87	)	$	(14.20	)
	basic and diluted
	The following weighted-average outstanding common stock equivalents were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive:
		Year Ended December 31,
		2014			2013			2012
	Convertible preferred stock		1,610,834			19,598,486			8,354,772
	Stock options to purchase common stock		2,572,729			1,666,036			1,563,217
	Unvested restricted stock units		9,715			—			—
	Common stock subject to repurchase		—			201,033			1,741,591
	Convertible preferred stock warrants		29,051			353,459			353,459
	Common stock warrants		318,666			—			—
			4,540,995			21,819,014			12,013,039

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Events	14.      Subsequent Events
	Underwritten Public Offering
	In February 2015, we completed an underwritten public offering in which we sold 3,450,000 shares of our common stock, which included 450,000 shares purchased by the underwriters pursuant to an option granted to them in connection with the offering, at a public offering price of $54.00 per share. The total proceeds that the Company received from the offering were approximately $175.1 million, net of underwriting discounts and commissions of approximately $11.2 million. After deducting offering expenses payable of approximately $0.6 million, net proceeds were $174.5 million.
	Lease Amendment
	On March 9, 2015, the Company entered into an amendment to its existing lease to obtain an additional office building with warehouse space. The amendment to this noncancelable lease commences on May 1, 2015 and co-terminates with the existing lease on April 30, 2019. At the end of the amended lease term, the Company has the option to extend the entire lease for two additional consecutive terms of five years. As specified in the amendment, the Company also increased the existing irrevocable letter of credit required by the lease by $287,000 for a total of $762,000 after giving effect for the scheduled April 2015 reduction of $119,000. Provided there has been no default on the lease, the Company may reduce the amount of the letter of credit by approximately $191,000 on each anniversary of the lease amendment commencement date. The amendment also provides for an additional tenant improvement allowance of up to $287,000. In addition, the Company is required to enter into a separate irrevocable letter of credit, for approved budgeted improvements.

Quarterly_Financial_Data_unaud

Quarterly Financial Data (unaudited)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Quarterly Financial Data (unaudited)	15. Quarterly Financial Data (unaudited)
	The following table presents certain unaudited quarterly financial information. This information has been prepared on the same basis as the audited financial statements and includes all adjustments (consisting only of normal recurring adjustments) necessary to present fairly the unaudited quarterly results of operations set forth herein. Net loss per share for all periods presented has been retroactively adjusted to reflect the 1-for-3.1345 reverse stock split effected on January 17, 2014.
	(in thousands, except share and per share data)
		2014
		March 31,			June 30,			September 30,			December 31,
	Operating expenses	$	10,339		$	13,661		$	15,835		$	16,943
	Net loss		(13,630	)		(13,585	)		(15,849	)		(16,738	)
	Net loss attributable to common stockholders		(18,438	)		(13,585	)		(15,849	)		(16,738	)
	Net loss per share applicable to	$	(0.85	)	$	(0.45	)	$	(0.50	)	$	(0.52	)
	common stockholders, basic and diluted
		2013
		March 31,			June 30,			September 30,			December 31,
	Operating expenses	$	6,747		$	8,247		$	7,761		$	9,525
	Net loss		(6,735	)		(8,590	)		(8,428	)		(11,317	)
	Net loss attributable to common stockholders		(8,205	)		(10,829	)		(12,590	)		(18,665	)
	Net loss per share applicable to	$	(2.84	)	$	(3.32	)	$	(3.48	)	$	(4.98	)
	common stockholders, basic and diluted
	Subsequent to the filing of its Form 10-Q for the third quarter of 2014, the Company determined that the reclassification of deferred offering costs, related to its initial public offering in February 2014, from other assets to additional paid-in capital, was not appropriately reflected in the Statements of Cash Flows for the three, six and nine months ended March 31, 2014, June 30, 2014, and September 30, 2014.  The Company assessed the materiality of this error, based on quantitative and qualitative factors, and concluded that it was not material to the previously issued interim financial statements taken as a whole. As such, it has corrected its Statement of Cash Flows for the year ended December 31, 2014 and will do so for the three, six and nine months ended March 31, 2014, June 30, 2014, and September 30, 2014, in its Form 10-Q filings during 2015. The revisions had no impact on net cash used in investing activities, net increase in cash and cash equivalents, or net loss per share in any period.  The following table reflects the revisions to the Statements of Cash Flows (in thousands):
		2014
		Three Months Ended						Six Months Ended						Nine Months Ended
		March 31,						June 30,						September 30,
		Previously Reported			As Adjusted			Previously Reported			As Adjusted			Previously Reported			As Adjusted
	Net cash used in operating activities	$	(8,336	)	$	(10,621	)	$	(18,865	)	$	(21,150	)	$	(29,895	)	$	(32,180	)
	Net cash provided by financing activities		121,754			124,039			121,764			124,049			182,274			184,559

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates
	The accompanying financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”). The preparation of the financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent liabilities and the reported amounts of expenses in the financial statements and the accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trial accruals, fair value of assets and liabilities, convertible preferred stock and related warrants, common stock, income taxes and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents
	The Company considers all highly liquid investments with original maturities of three months or less from the date of purchase to be cash equivalents. Cash equivalents consist primarily of amounts invested in money market accounts and corporate bonds.
Short-Term Investments	Short-Term Investments
	All investments have been classified as “available-for-sale” and are carried at estimated fair value as determined based upon quoted market prices or pricing models for similar securities. Management determines the appropriate classification of its investments in debt securities at the time of purchase and reevaluates such designation as of each balance sheet date. Unrealized gains and losses are excluded from earnings and are reported as a component of comprehensive loss. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on available-for-sale securities are included in interest income and other expense, net, respectively. The cost of securities sold is based on the specific-identification method. Interest on marketable securities is included in interest income.
Concentration of Credit Risk and Other Risks and Uncertainties	Concentration of Credit Risk and Other Risks and Uncertainties
	Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents, and short-term investments. The Company’s cash, cash equivalents, and short-term investments are held by financial institutions that management believes are of high credit quality. The Company’s investment policy limits investments to fixed income securities denominated and payable in U.S. dollars such as U.S. government obligations, money market instruments and funds, corporate bonds, and asset-backed securities and places restrictions on maturities and concentrations by type and issuer. Such deposits may, at times, exceed federally insured limits. The Company has not experienced any losses on its deposits of cash and cash equivalents and its accounts are monitored by management to mitigate risk. The Company is exposed to credit risk in the event of default by the financial institutions holding its cash and cash equivalents and corporate bond issuers to the extent recorded in the balance sheets.
Property and Equipment	Property and Equipment
	Property and equipment are stated at cost, less accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the respective assets. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the balance sheet and the resulting gain or loss, if any, is reflected in operations.
	The useful lives of the property and equipment are as follows:
	Research and development equipment	5 years
	Furniture and office equipment	5 years
	Computer equipment	3 years
	Software	3 years
	Leasehold improvements	Shorter of lease term or estimated useful life
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets
	The Company evaluates its long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows expected to result from the use of the asset and its eventual disposition. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset. The Company has not recorded impairment of any long-lived assets since inception.
Restricted Cash	Restricted Cash
	Restricted cash includes a money market account with one of the Company’s financial institutions as collateral for its obligations under its facility lease of the Company’s corporate headquarters in Novato, California. Restricted cash also includes a savings account associated with a credit card agreement at one of the Company’s financial institutions.
Deferred Offering Costs	Deferred Offering Costs
	Deferred offering costs, which primarily consist of direct incremental legal and accounting fees relating to the IPO and the Company’s underwritten public offering in July 2014, were initially capitalized. IPO deferred offering costs of $3.3 million were subsequently offset against IPO proceeds upon the closing of the offering in February 2014. Underwritten public offering costs of $444,000 were subsequently offset against offering proceeds upon the completion of the underwritten public offering in July 2014. As of December 31, 2014 and 2013, deferred offering costs of $0 and $2.3 million, respectively, were capitalized and included in other assets on the balance sheet.
Accruals of Research and Development Costs	Accruals of Research and Development Costs
	The Company records accruals for estimated costs of research, preclinical and clinical studies and manufacturing development. These costs are a significant component of the Company’s research and development expenses. A substantial portion of the Company’s ongoing research and development activities are conducted by third-party service providers, including contract research organizations. The Company accrues the costs incurred under its agreements with these third parties based on actual work completed in accordance with agreements established with these third parties. The Company determines the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of the services and the agreed-upon fee to be paid for such services. The Company makes significant judgments and estimates in determining the accrual balance in each reporting period. As actual costs become known, the Company adjusts its accruals. The Company has not experienced any material deviations between accrued clinical trial expenses and actual clinical trial expenses. However, actual services performed, number of patients enrolled, and the rate of patient enrollment may vary from the Company’s estimates, resulting in adjustments to clinical trial expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations.
Leases	Leases
	The Company enters into lease agreements for its office and laboratory facilities. These leases are classified as operating leases. Rent expense is recognized on a straight-line basis over the term of the lease and, accordingly, the Company records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. Incentives granted under the Company’s facilities leases, including allowances to fund leasehold improvements, are deferred and are recognized as adjustments to rental expense on a straight-line basis over the term of the lease.
Convertible Preferred Stock Warrant Liability	Convertible Preferred Stock Warrant Liability
	Warrants for shares that are either puttable or that are contingently redeemable are classified as liabilities on the balance sheets. The warrants were subject to remeasurement at each balance sheet date, with changes in fair value recognized as a component of other expense, net. The Company continued to adjust the liability for changes in fair value until the completion of its IPO, whereby the warrants were converted to common stock warrants that are no longer subject to remeasurement.
Convertible Preferred Stock	Convertible Preferred Stock
	The Company initially records all shares of convertible preferred stock net of offering costs on the dates of issuance, which represents the carrying value. Only the passage of time was required for Series A convertible preferred stock to become redeemable; accordingly, the Company was accreting the carrying value of Series A convertible preferred stock to its redemption value over the period from the date of issuance to June 16, 2017 (the earliest redemption date). The convertible preferred stock was classified outside of stockholders’ equity (deficit) on the accompanying balance sheets, as Series A convertible preferred stock could have been redeemed and as events triggering the liquidation preferences for both classes of convertible preferred stock were not solely within the Company’s control. In connection with the closing of Company’s IPO in February 2014, all of the Company’s outstanding convertible preferred stock was converted into common stock.
Comprehensive Loss	Comprehensive Loss
	Comprehensive loss is the change in stockholders’ equity (deficit) from transactions and other events and circumstances other than those resulting from investments by stockholders and distributions to stockholders. The Company’s other comprehensive loss is comprised of unrealized gains and losses on investments in available-for-sale securities.
Research and Development	Research and Development
	Research and development costs are expensed as incurred and consist of salaries and benefits, stock-based compensation expense, lab supplies and facility costs, as well as fees paid to other nonemployees and entities that conduct certain research and development activities on the Company’s behalf. Amounts incurred in connection with license agreements are also included in research and development expense. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed.
Stock-Based Compensation	Stock-Based Compensation
	Stock-based awards issued to employees, including stock options, are recorded at fair value as of the grant date using the Black-Scholes option-pricing model and recognized as expense on a straight-line basis over the employee’s requisite service period (generally the vesting period). Because noncash stock compensation expense is based on awards ultimately expected to vest, it is reduced by an estimate for future forfeitures. Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from estimates.
Income Taxes	Income Taxes
	The Company uses the liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. The Company must then assess the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance.
	The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.
Net Loss per Share Attributable to Common Stockholders	Net Loss per Share Attributable to Common Stockholders
	Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. The net loss attributable to common stockholders is calculated by adjusting the net loss of the Company for the accretion on the Series A convertible preferred stock and cumulative dividends on Series A and B convertible preferred stock. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since the effects of potentially dilutive securities are antidilutive.
Reverse Stock Split	Reverse Stock Split
	In January 2014, the Company’s board of directors and its stockholders approved an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of shares of its common stock on a 1-for-3.1345 basis (the “Reverse Stock Split”). The par values and the authorized shares of the common and convertible preferred stock were not adjusted as a result of the Reverse Stock Split, nor were the outstanding shares of preferred stock. All issued and outstanding common stock and related per share amounts contained in the financial statements have been retroactively adjusted to reflect this Reverse Stock Split for all periods presented. A proportional adjustment to the conversion ratio for each series of convertible preferred stock was also effected in connection with the reverse split. The Reverse Stock Split was effected on January 17, 2014.
Recent Accounting Pronouncements	Recent Accounting Pronouncements
	In June 2014, the FASB issued ASU 2014-10, Development Stage Enterprises (Topic 915): Elimination of Certain Financial Reporting Requirements, Including an Amendment to Variable Interest Entities Guidance in Topic 810, Consolidation (ASU 2014-10). ASU 2014-10 removes the definition of a development stage entity from the Master Glossary of the Accounting Standards Codification, thereby removing the financial reporting distinction between development stage entities and other reporting entities from U.S. GAAP. ASU 2014-10 also eliminates the requirements for development stage entities to (1) present inception-to-date information in the statements of income, cash flows, and shareholder equity, (2) label the financial statements as those of a development stage entity, (3) disclose a description of the development stage activities in which the entity is engaged, and (4) disclose in the first year in which the entity is no longer a development stage entity that in prior years it had been in the development stage. The amendments also clarify that the guidance in Topic 275, Risks and Uncertainties, is applicable to entities that have not commenced planned principal operations. The Company elected to early adopt the provisions of ASU 2014-10, as reflected in the financial statements.
	In August 2014, the FASB issued ASU No. 2014-15, Presentation of Financial Statements — Going Concern (Subtopic 205-40): Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern, which requires management to evaluate, in connection with preparing financial statements for each annual and interim reporting period, whether there are conditions or events, considered in the aggregate, that raise substantial doubt about an entity’s ability to continue as a going concern within one year after the date that the financial statements are issued and provide related disclosures. This ASU will be effective for the Company in fiscal year 2016. Early adoption is permitted. We are currently assessing the future impact of this ASU in the financial statements.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Summary of Useful Lives of Property and Equipment	The useful lives of the property and equipment are as follows:
	Research and development equipment	5 years
	Furniture and office equipment	5 years
	Computer equipment	3 years
	Software	3 years
	Leasehold improvements	Shorter of lease term or estimated useful life

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Summary of Financial Assets and Liabilities Measured on Recurring Basis	The following table sets forth the fair value of the Company’s financial assets and liabilities remeasured on a recurring basis based on the three-tier fair value hierarchy (in thousands):
		31-Dec-14
		Level 1			Level 2		Level 3		Total
	Financial Assets:
	Money market funds	$	8,627		$	—	$	—	$	8,627
	Corporate bonds		—			152,942		—		152,942
	Asset backed securities		—			9,542		—		9,542
	U.S. Government agency securities		—			4,485		—		4,485
	Other		—			209		—		209
	Total financial assets	$	8,627		$	167,178	$	—	$	175,805
		31-Dec-13
		Level 1			Level 2		Level 3		Total
	Financial Assets:
	Money market funds	$	6,847		$	—	$	—	$	6,847
	Commercial paper		—			1,000		—		1,000
	Corporate bonds		—			44,950		—		44,950
	Total financial assets	$	6,847		$	45,950	$	—	$	52,797
	Financial Liabilities:
	Convertible preferred stock warrant liability	$	—		$	—	$	3,419	$	3,419
	Total financial liabilities	$	—		$	—	$	3,419	$	3,419
Summary of Changes in Estimated Fair Value	The following table sets forth a summary of the changes in the estimated fair value of the Company’s convertible preferred stock warrants, which were measured at fair value on a recurring basis until their conversion to common stock warrants and related reclassification to additional paid-in capital (in thousands):
		Year Ended December 31,
		2014			2013
	Fair value, beginning of year	$	3,419		$	518
	Change in fair value recorded as a loss in other expense, net		3,324			2,901
	Reclassification of warrant liability to additional paid-in		(6,743	)		—
	capital
	Fair value, end of year	$	—		$	3,419

Balance_Sheet_Components_Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2014
Investments Debt And Equity Securities [Abstract]
Summary of Fair Values of Cash Equivalents, Restricted Cash and Short-Term Investments Classified as Available-for-Sale Securities	The fair values of cash equivalents, restricted cash and short-term investments classified as available-for-sale securities, consisted of the following (in thousands):
		31-Dec-14
						Gross Unrealized
		Amortized				Gains				Losses			Estimated Fair Value
		Cost
	Money market funds classified as cash equivalents	$	8,627			$	—			$	—		$	8,627
	Corporate bonds classified as cash equivalents		3,806				1				—			3,807
	Commercial Paper classified as short-term investments		—				—				—			—
	Corporate bonds classified as short-term investments		149,303				4				(172	)		149,135
	Asset backed securities		9,546				—				(4	)		9,542
	U.S. Government agency securities classified as short-term		4,488				1				(4	)		4,485
	investments
	Other		209				—				—			209
	Total	$	175,979			$	6			$	(180	)	$	175,805
		31-Dec-13
						Gross Unrealized
		Amortized Cost				Gains				Losses			Estimated Fair Value
	Money market funds classified as cash equivalents	$	6,847			$	—			$	—		$	6,847
	Commercial Paper classified as short-term investments		1,000				—				—			1,000
	Corporate bonds classified as short-term investments		44,939				17				(6	)		44,950
	Total	$	52,786			$	17			$	(6	)	$	52,797
Summary of Property and Equipment	Property and equipment, net consists of the following (in thousands):
			December 31,
			2014				2013
	Research and development equipment		$	712			$	277
	Furniture and office equipment			572				385
	Computer equipment			268				248
	Software			821				58
	Leasehold improvements			2,141				1,154
	Property and equipment, gross			4,514				2,122
	Less accumulated depreciation			(1,481	)			(797	)
	Property and equipment, net		$	3,033			$	1,325
Accrued Liabilities	Accrued liabilities consist of the following (in thousands):
			December 31,
			2014				2013
	Research and clinical trial expenses		$	2,703			$	1,886
	Payroll and related expenses			4,205				2,360
	Other			667				160
	Total accrued liabilities		$	7,575			$	4,406

Convertible_Preferred_Stock_Wa1

Convertible Preferred Stock Warrants and Common Stock Warrants (Tables)	12 Months Ended
	Dec. 31, 2014
Warrants And Rights Note Disclosure [Abstract]
Summary of Outstanding Warrants	As of December 31, 2014, outstanding warrants consisted of the following:
	Common Stock Warrants:	Number of Warrants		Date Issued	Term	Exercise
						Price
	Common stock		83,167	Jun-10	10 years	$	3.006
	Common stock		174,651	Feb-11	10 years		3.006
	Common stock		66,533	Jun-11	10 years		3.006
	Total common stock warrants		324,351
	As of December 31, 2013, outstanding warrants consisted of the following:
	Convertible Preferred Stock Warrants:	Number of Warrants		Date Issued	Term	Exercise
						Price
	Series A		241,803	Jun-10	10 years	$	1.034
	Series A		592,417	Feb-11	10 years		1.034
	Series A		193,442	Jun-11	10 years		1.034
	Total convertible preferred stock warrants		1,027,662

Common_Stock_Tables

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2014
Warrants And Rights Note Disclosure [Abstract]
Schedule of Reserved Shares of Common Stock for Future Issuance	The Company had reserved shares of common stock, on an as-converted basis, for future issuance as follows:
		December 31,
		2014		2013
	Convertible preferred stock		—		19,598,486
	Convertible preferred stock warrants		—		353,459
	Common stock warrants		324,351		—
	Employee stock purchase plan		600,000		—
	2011 equity incentive plan		1,960,225		2,228,883
	2014 equity incentive plan		810,250		—
	Shares available for future stock option grants		1,439,750		799,963
			5,134,576		22,980,791

Convertible_Preferred_Stock_Ta

Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Schedule of Convertible Preferred Stock	Under the Company’s Amended and Restated Certificate of Incorporation, as amended, the Company is authorized to issue two classes of shares: preferred and common stock. The preferred stock is issuable in series, and the Company’s board of directors is authorized to determine the rights, preferences and terms of each series. As of December 31, 2014, no shares of preferred stock were outstanding. As of December 31, 2013, convertible preferred stock consisted of the following (in thousands, except share and per share amounts):
		As of December 31, 2013
	Convertible Preferred Stock:	Shares Authorized		Shares Issued and Outstanding		Proceeds Net of Issuance Costs		Aggregate Liquidation Preference
	Series A		35,377,556		34,349,894	$	33,623	$	38,063
	Series B		27,081,680		27,081,680		73,929		76,739
	Total convertible preferred stock		62,459,236		61,431,574	$	107,552	$	114,802
	The preferred stock is issuable in series, and the Company’s board of directors is authorized to determine the rights, preferences and terms of each series. As of December 31, 2014, no shares of preferred stock were outstanding. As of December 31, 2013, convertible preferred stock consisted of the following (in thousands, except share and per share amounts):

StockBased_Awards_Tables

Stock-Based Awards (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Option Activity Under 2011 Plan and 2014 Plan	Stock Option Activity
	A summary of option activity under the 2011 Plan and the 2014 Plan and related information are as follows:
			Options Outstanding
			Number of Options				Weighted- Average Exercise Price				Weighted-Average Remaining Contractual Term (Years)				Aggregate Intrinsic Value
															(In thousands)
	Outstanding — December 31, 2011			1,451,580			$	0.31				9.88			$	—
	Shares reserved			—				—
	Options granted			499,278				0.81
	Options exercised			(420,308	)			0.31
	Options cancelled			(9,583	)			0.31
	Outstanding — December 31, 2012			1,520,967				0.47				9.11			$	2,038
	Options granted			1,267,797				5.8
	Options exercised			(307,366	)			0.55
	Options cancelled			(252,515	)			1.26
	Outstanding — December 31, 2013			2,228,883				3.41			8.91				$	19,468
	Shares reserved			—				—
	Options granted			932,555				41.12
	Options exercised			(305,090	)			2.14
	Options cancelled			(116,873	)			8.92
	Outstanding — December 31, 2014			2,739,475				16.15				8.43			$	79,840
	Vested and exercisable — December 31, 2014			802,189				2.65			7.55				$	33,072
	Vested and expected to vest — December 31,			2,666,378			$	15.86				8.41			$	78,390
	2014
Summary of Options Outstanding and Exercisable by Exercise Price	The options outstanding and exercisable by exercise price as of December 31, 2014 are as follows:
					Options Outstanding								Options Exercisable
	Exercise Price				Numbers Outstanding				Weighted-Average Remaining Contractual Term (in Years)				Number Exercisable				Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (in Years)
	$	0.31				667,554				6.88				472,481			$	0.31	6.88
	$0.81 to $1.82					453,552				7.98				136,284				1.33	7.98
	$4.07 to $6.86					523,169				8.81				125,181				6.59	8.81
	$11.19 to $21.00					315,950				9.01				68,243				14.26	9.01
	$33.55 to $34.93					249,500				9.4				—				—	—
	$40.37 to $49.15					203,750				9.8				—				—	—
	$53.03 to $58.02					326,000				9.43				—				—	—
						2,739,475				8.43				802,189			$	2.65	7.55
Summary of Stock-Based Compensation Recognized	Total stock-based compensation recognized was as follows (in thousands):
			Year Ended December 31,
			2014				2013				2012
	Research and development		$	4,116			$	293			$	130
	General and administrative			1,278				364				761
	Total stock-based compensation expense		$	5,394			$	657			$	891
Fair Value of Stock Option Awards Estimated Using Black-Scholes Option-Pricing Model	The fair value of stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following weighted-average assumptions:
			Year Ended December 31,
			2014				2013				2012
	Expected term		6.23 years				6.25 years				6.25 years
	Expected volatility			70%				74%				67%
	Risk-free interest rate			1.90%			0.7% to 1.6%					0.60%
	Expected dividend rate			0.00%				0.00%				0.00%

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Effective Tax Rate of Provision for Income Taxes from Federal Statutory Rate	The effective tax rate of our provision for income taxes differs from the federal statutory rate as follows:
		Year Ended December 31,
		2014				2013				2012
	Federal statutory income tax rate		34		%		34		%		34		%
	State income taxes, net of federal benefit		4.6				5				6
	Federal tax credits		6.7				8.2				8.6
	Nondeductible permanent items		(1.2	)			(2.9	)			—
	Stock compensation		(1.4	)			(0.6	)			(1.9	)
	Change in valuation allowance		(42.7	)			(44.6	)			(46.0	)
	Uncertain tax positions		—				0.9				(0.7	)
	Provision for income taxes		—				—				—
Schedule of Tax Effect of Temporary Differences to Significant Portions of Deferred Tax Assets	The tax effect of temporary differences that give rise to significant portions of the deferred tax assets is presented below (in thousands):
		Year Ended December 31,
		2014				2013
	Deferred tax assets:
	Net operating loss carryforwards	$	32,500			$	16,352
	Tax credits		16,737				8,692
	Stock options		1,058				—
	Accruals and reserves		476				591
	Fixed assets and intangibles		455				65
	Other		113				—
	Total deferred tax assets		51,339				25,700
	Valuation allowance		(51,339	)			(25,700	)
	Net deferred tax assets	$	—			$	—
Unrecognized Tax Benefits	A reconciliation of the Company’s unrecognized tax benefits for the years ended December 31, 2014 and 2013 is as follows (in thousands):
		December 31,
		2014				2013				2012
	Balance at beginning of year	$	3,725			$	1,551			$	324
	Additions based on tax positions related to current year		3,550				2,169				1,227
	Additions (reductions) for tax positions of prior years		—				5				—
	Balance at end of year	$	7,275			$	3,725			$	1,551

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Lease Payments under Non-Cancellable Operating Leases Arrangements	As of December 31, 2014, the aggregate future minimum lease payments under the noncancelable operating lease arrangements are as follows (in thousands):
	Year Ending December 31,
	2015	$	1,173
	2016		1,305
	2017		1,335
	2018		1,366
	2019		560
	Thereafter		—
		$	5,739
	*- Includes additional lease payments under the lease amendment entered into in March 2015, as detailed in Note 14 “Subsequent Events”

Net_Loss_per_Share_Attributabl1

Net Loss per Share Attributable to Common Stockholders (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss per Share	The following table sets forth the computation of the basic and diluted net loss per share attributable to common stockholders during the years ended December 31, 2014, 2013 and 2012 (in thousands, except share and per share data):
		Year Ended December 31,
		2014			2013			2012
	Numerator:
	Net loss	$	(59,802	)	$	(35,070	)	$	(16,334	)
	Accretion and dividends on convertible preferred stock		(4,808	)		(15,219	)		(3,227	)
	Net loss attributable to common stockholders	$	(64,610	)	$	(50,289	)	$	(19,561	)
	Denominator:
	Weighted-average common shares outstanding		28,755,758			3,583,522			3,118,798
	Less: weighted-average unvested common shares subject to		—			(201,033	)		(1,741,591	)
	repurchase
	Weighted-average shares used to compute net loss per		28,755,758			3,382,489			1,377,207
	share attributable to common stockholders, basic and diluted
	Net loss per share attributable to common stockholders,	$	(2.25	)	$	(14.87	)	$	(14.20	)
	basic and diluted
Outstanding Shares of Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share	The following weighted-average outstanding common stock equivalents were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive:
		Year Ended December 31,
		2014			2013			2012
	Convertible preferred stock		1,610,834			19,598,486			8,354,772
	Stock options to purchase common stock		2,572,729			1,666,036			1,563,217
	Unvested restricted stock units		9,715			—			—
	Common stock subject to repurchase		—			201,033			1,741,591
	Convertible preferred stock warrants		29,051			353,459			353,459
	Common stock warrants		318,666			—			—
			4,540,995			21,819,014			12,013,039

Quarterly_Financial_Data_unaud1

Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Summary of Unaudited Quarterly Financial Information	The following table presents certain unaudited quarterly financial information. This information has been prepared on the same basis as the audited financial statements and includes all adjustments (consisting only of normal recurring adjustments) necessary to present fairly the unaudited quarterly results of operations set forth herein. Net loss per share for all periods presented has been retroactively adjusted to reflect the 1-for-3.1345 reverse stock split effected on January 17, 2014.
	(in thousands, except share and per share data)
		2014
		March 31,			June 30,			September 30,			December 31,
	Operating expenses	$	10,339		$	13,661		$	15,835		$	16,943
	Net loss		(13,630	)		(13,585	)		(15,849	)		(16,738	)
	Net loss attributable to common stockholders		(18,438	)		(13,585	)		(15,849	)		(16,738	)
	Net loss per share applicable to	$	(0.85	)	$	(0.45	)	$	(0.50	)	$	(0.52	)
	common stockholders, basic and diluted
		2013
		March 31,			June 30,			September 30,			December 31,
	Operating expenses	$	6,747		$	8,247		$	7,761		$	9,525
	Net loss		(6,735	)		(8,590	)		(8,428	)		(11,317	)
	Net loss attributable to common stockholders		(8,205	)		(10,829	)		(12,590	)		(18,665	)
	Net loss per share applicable to	$	(2.84	)	$	(3.32	)	$	(3.48	)	$	(4.98	)
	common stockholders, basic and diluted
Revised Statement of Cash Flows	The following table reflects the revisions to the Statements of Cash Flows (in thousands):
		2014
		Three Months Ended						Six Months Ended						Nine Months Ended
		March 31,						June 30,						September 30,
		Previously Reported			As Adjusted			Previously Reported			As Adjusted			Previously Reported			As Adjusted
	Net cash used in operating activities	$	(8,336	)	$	(10,621	)	$	(18,865	)	$	(21,150	)	$	(29,895	)	$	(32,180	)
	Net cash provided by financing activities		121,754			124,039			121,764			124,049			182,274			184,559

Organization_and_Basis_of_Pres1

Organization and Basis of Presentation - Additional Information (Details) (USD $)	1 Months Ended	12 Months Ended	1 Months Ended	0 Months Ended
	Jul. 31, 2014	Dec. 31, 2014	Feb. 28, 2015	Feb. 05, 2014	Dec. 31, 2013	Jan. 30, 2014
Organization And Nature Of Business [Line Items]
Cumulative net losses		$119,400,000
Net proceeds from issuance of common stock		121,711,000
Offering expenses	400,000
Common stock, shares issued	1,613,879	31,934,682			31,934,682
Purchase of additional shares by underwriters	302,602
Common stock issued price per share	$40
Common stock sold by existing shareholders	706,072
Proceeds from public offering before offering expenses	60,700,000
Underwriting discounts and commissions	3,900,000
Proceeds from public offering, net	60,200,000
Subsequent Event
Organization And Nature Of Business [Line Items]
Offering expenses			600,000
Common stock, shares issued			3,450,000
Purchase of additional shares by underwriters			450,000
Common stock issued price per share			$54
Proceeds from public offering before offering expenses			175,100,000
Underwriting discounts and commissions			11,200,000
Proceeds from public offering, net			174,500,000
IPO
Organization And Nature Of Business [Line Items]
Effective date of registration		30-Jan-14
Public offering price of the shares sold						$21
Common stock issued in initial public offering				6,624,423
Net proceeds from issuance of common stock				129,400,000
Underwriting discounts and commissions				9,700,000
Offering expenses				3,300,000
Cash dividends paid to preferred stockholders				4,300,000
Net proceeds from initial public offering				121,700,000
Number of preferred stock converted into common stock				19,598,486
IPO | Subsequent Event
Organization And Nature Of Business [Line Items]
Public offering price of the shares sold			$54
Common stock issued in initial public offering			3,450,000
Net proceeds from issuance of common stock			175,100,000
Underwriting discounts and commissions			11,200,000
Offering expenses			600,000
Net proceeds from initial public offering			$174,500,000
IPO | Over-Allotment Option
Organization And Nature Of Business [Line Items]
Common stock issued in initial public offering				864,054
United States of America
Organization And Nature Of Business [Line Items]
Number of reportable segments		1

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Summary of Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2014
Research and Development Equipment
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	5 years
Furniture and Office Equipment
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	5 years
Computer Equipment
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	3 years
Software
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	3 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property plant and equipment useful life	Shorter of lease term or estimated useful life

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Additional Information (Details) (USD $)	0 Months Ended	12 Months Ended
	Jan. 17, 2014	Dec. 31, 2014	Jul. 31, 2014	Feb. 05, 2014	Dec. 31, 2013
Summary Of Significant Accounting Policies [Line Items]
Offering expenses			$400,000
Convertible preferred stock redemption date		16-Jun-17
Reverse stock split ratio on preferred and common stock	3.1345
IPO
Summary Of Significant Accounting Policies [Line Items]
Offering expenses				3,300,000
Prepaid And Other Current Assets | Costs Capitalized
Summary Of Significant Accounting Policies [Line Items]
Offering expenses		0			2,300,000
Underwritten Offering Costs
Summary Of Significant Accounting Policies [Line Items]
Offering expenses			$444,000

Fair_Value_Measurements_Summar

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured on Recurring Basis (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Financial Assets:
Fair value	$175,805	$52,797
Financial Liabilities:
Fair value		3,419
Money Market Funds
Financial Assets:
Fair value	8,627	6,847
Asset Backed Securities
Financial Assets:
Fair value	9,542
Other
Financial Assets:
Fair value	209
Commercial Paper
Financial Assets:
Fair value		1,000
Level 1
Financial Assets:
Fair value	8,627	6,847
Level 1 | Money Market Funds
Financial Assets:
Fair value	8,627	6,847
Level 2
Financial Assets:
Fair value	167,178	45,950
Level 2 | Asset Backed Securities
Financial Assets:
Fair value	9,542
Level 2 | Other
Financial Assets:
Fair value	209
Level 2 | Commercial Paper
Financial Assets:
Fair value		1,000
Level 3
Financial Liabilities:
Fair value		3,419
Corporate Bonds
Financial Assets:
Fair value	152,942	44,950
Corporate Bonds | Level 2
Financial Assets:
Fair value	152,942	44,950
US Government Corporations and Agencies Securities
Financial Assets:
Fair value	4,485
US Government Corporations and Agencies Securities | Level 2
Financial Assets:
Fair value	4,485
Convertible Preferred Stock Warrant Liability
Financial Liabilities:
Fair value		3,419
Convertible Preferred Stock Warrant Liability | Level 3
Financial Liabilities:
Fair value		$3,419

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Details) (USD $)	0 Months Ended
	Dec. 31, 2013	Jan. 30, 2014
Back Solve Method
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Estimated fair value of common stock, per share	$12.14
Expected volatility	70.00%
Expected time	5 months 5 days
Expected risk free interest rate	0.11%
Black-Scholes Option Pricing Model
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Estimated fair value of common stock, per share		$21
Expected volatility		70.00%
Expected time		7 years
Expected risk free interest rate		2.19%

Fair_Value_Measurements_Summar1

Fair Value Measurements - Summary of Changes in Estimated Fair Value (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value Disclosures [Abstract]
Fair value, beginning of year	$3,419	$518
Change in fair value recorded as a loss in other expense, net	3,324	2,901
Reclassification of warrant liability to additional paid-in capital	-6,743
Fair value, end of year		$3,419

Balance_Sheet_Components_Summa

Balance Sheet Components - Summary of Fair Values of Cash Equivalents, Restricted Cash and Short-Term Investments Classified as Available-for-Sale Securities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$175,979	$52,786
Gross Unrealized Gains	6	17
Gross Unrealized Losses	-180	-6
Estimated Fair Value	175,805	52,797
Money Market Funds Classified as Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	8,627	6,847
Estimated Fair Value	8,627	6,847
Corporate Bonds Classified as Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	3,806
Gross Unrealized Gains	1
Estimated Fair Value	3,807
Commercial Paper Classified as Short-Term Investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		1,000
Estimated Fair Value		1,000
Corporate Bonds Classified as Short-Term Investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	149,303	44,939
Gross Unrealized Gains	4	17
Gross Unrealized Losses	-172	-6
Estimated Fair Value	149,135	44,950
Asset Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,546
Gross Unrealized Losses	-4
Estimated Fair Value	9,542
Other
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	209
Estimated Fair Value	209
US Government Corporations and Agencies Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	4,488
Gross Unrealized Gains	1
Gross Unrealized Losses	-4
Estimated Fair Value	$4,485

Balance_Sheet_Components_Addit

Balance Sheet Components - Additional Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Available For Sale Securities [Line Items]
Significant realized gains or losses on available-for-sale securities	$0
Depreciation and amortization	$684,000	$444,000	$313,000
Maximum
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities remaining contractual maturities	2 years

Balance_Sheet_Components_Summa1

Balance Sheet Components - Summary of Property and Equipment (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Property Plant And Equipment [Line Items]
Property and equipment, gross	$4,514	$2,122
Less accumulated depreciation	-1,481	-797
Property and equipment, net	3,033	1,325
Research and Development Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	712	277
Furniture and Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	572	385
Computer Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	268	248
Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	821	58
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$2,141	$1,154

Balance_Sheet_Components_Accru

Balance Sheet Components - Accrued Liabilities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Payables And Accruals [Abstract]
Research and clinical trial expenses	$2,703	$1,886
Payroll and related expenses	4,205	2,360
Other	667	160
Total accrued liabilities	$7,575	$4,406

License_and_Research_Agreement1

License and Research Agreements - Additional Information (Details)

Dec. 31, 2014

Jul. 31, 2014

Dec. 31, 2013

Sep. 30, 2010

Sep. 30, 2010

Dec. 31, 2014

Dec. 31, 2014

Nov. 30, 2010

Dec. 31, 2014

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2012

Dec. 31, 2014

Sep. 30, 2010

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

USD ($)

USD ($)

Nobelpharma License Agreement

Nobelpharma License Agreement

Nobelpharma License Agreement

Nobelpharma License Agreement

Saint Louis University License Agreement

Saint Louis University License Agreement

H I B M Research Group

H I B M Research Group

H I B M Research Group

St. Jude Children's Research Hospital License Agreement

St. Jude Children's Research Hospital License Agreement

Baylor Research Institute License Agreement

Baylor Research Institute License Agreement

Baylor Research Institute License Agreement

Baylor Research Institute License Agreement

Kyowa Hakko Kirin Collaboration

Kyowa Hakko Kirin Collaboration

USD ($)

JPY (¥)

USD ($)

JPY (¥)

USD ($)

USD ($)

USD ($)

Maximum

Development and Approval Milestones

USD ($)

Maximum

USD ($)

USD ($)

Development Milestones

Sales Milestones

License Agreement

License Agreement

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

Research And Development [Line Items]

License agreement

$110,500

¥ 10,000,000

$10,000

$100,000

$25,000

$10,000

$250,000

$4,600,000

$402,000

Common stock, shares issued

31,934,682

1,613,879

31,934,682

76,567

76,567

Remaining total aggregate payments

1,700,000

200,000,000

Future contingent milestone payments

300,000

10,500,000

7,500,000

Payments for royalty on net sales

1.00%

1.00%

Payment of option exercise fee

750,000

License agreement receivable

1,300,000

370,000

Accrued liabilities

$7,575,000

$4,406,000

$35,000

$435,000

Convertible_Preferred_Stock_Wa2

Convertible Preferred Stock Warrants and Common Stock Warrants - Additional Information (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 30, 2014	Jun. 30, 2011	Feb. 11, 2011
Class Of Warrant Or Right [Line Items]
Fair value of warrants		$3,400		$6,700	$203
Revaluation of convertible preferred stock warrant liability	3,324	2,901	302
William Aliski | Convertible Note and Warrant Purchase Agreements
Class Of Warrant Or Right [Line Items]
Amount borrowed from officers						250
John Klock | Convertible Note and Warrant Purchase Agreements
Class Of Warrant Or Right [Line Items]
Amount borrowed from officers						$1,500
Convertible Preferred Stock
Class Of Warrant Or Right [Line Items]
Number of warrants		1,027,662			1,027,662
Exercise price					$1.03

Convertible_Preferred_Stock_Wa3

Convertible Preferred Stock Warrants and Common Stock Warrants - Summary of Outstanding Warrants (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Jun. 30, 2011
Common Stock
Class Of Warrant Or Right [Line Items]
Number of warrants	324,351
Common Stock | June 2010 Common Stock Warrant [Member]
Class Of Warrant Or Right [Line Items]
Number of warrants	83,167
Date Issued	2010-06
Term	10 years
Exercise price	$3.01
Common Stock | February 2011 Common Stock Warrant [Member]
Class Of Warrant Or Right [Line Items]
Number of warrants	174,651
Date Issued	2011-02
Term	10 years
Exercise price	$3.01
Common Stock | June 2011 Common Stock Warrant [Member]
Class Of Warrant Or Right [Line Items]
Number of warrants	66,533
Date Issued	2011-06
Term	10 years
Exercise price	$3.01
Convertible Preferred Stock
Class Of Warrant Or Right [Line Items]
Number of warrants		1,027,662	1,027,662
Exercise price			$1.03
Convertible Preferred Stock | June 2010 Series A Convertible Preferred Stock Warrant
Class Of Warrant Or Right [Line Items]
Number of warrants		241,803
Date Issued		2010-06
Term		10 years
Exercise price		$1.03
Convertible Preferred Stock | February 2011 Series A Convertible Preferred Stock Warrant
Class Of Warrant Or Right [Line Items]
Number of warrants		592,417
Date Issued		2011-02
Term		10 years
Exercise price		$1.03
Convertible Preferred Stock | June 2011 Series A Convertible Preferred Stock Warrants
Class Of Warrant Or Right [Line Items]
Number of warrants		193,442
Date Issued		2011-06
Term		10 years
Exercise price		$1.03

Common_Stock_Additional_Inform

Common Stock - Additional Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Dividends Common Stock [Abstract]
Common stock dividends declared	$0

Common_Stock_Reserved_Shares_o

Common Stock - Reserved Shares of Common Stock for Future Issuance (Details)	Dec. 31, 2014	Dec. 31, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Reserved shares of common stock for future issuance	5,134,576	22,980,791
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Reserved shares of common stock for future issuance	1,439,750	799,963
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Reserved shares of common stock for future issuance	600,000
2011 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Reserved shares of common stock for future issuance	1,960,225	2,228,883
2014 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Reserved shares of common stock for future issuance	810,250
Common Stock Warrants
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Reserved shares of common stock for future issuance	324,351
Convertible Preferred Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Reserved shares of common stock for future issuance		19,598,486
Convertible Preferred Stock Warrants
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Reserved shares of common stock for future issuance		353,459

Convertible_Preferred_Stock_Ad

Convertible Preferred Stock - Additional Information (Details) (USD $)	0 Months Ended	12 Months Ended			1 Months Ended
	Jan. 17, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 28, 2014
Temporary Equity [Line Items]
Preferred stock, shares outstanding		0	0
Amount of dividends in arrears paid			$4,000,000
Convertible preferred stock redemption date		16-Jun-17
Preferred stock convertible basis, description		In connection with the reverse split which was effective on January 17, 2014, all shares of preferred stock were convertible into common stock on a 3.1345-for-one basis.
Reverse stock split ratio on preferred and common stock	3.1345
IPO
Temporary Equity [Line Items]
Conversion of preferred stock into common stock, shares					19,598,486
Series A Preferred Stock
Temporary Equity [Line Items]
Dividends per share		0.062
Amount of dividends declared and paid				2,070,000
Issuance of convertible preference stock, per share		1.034
Convertible preferred stock redemption date		16-Jun-17
Series A Preferred Stock | IPO
Temporary Equity [Line Items]
Amount of dividends in arrears paid			4,000,000		4,300,000
Series B Convertible Preferred Stock
Temporary Equity [Line Items]
Dividends per share		0.062
Issuance of convertible preference stock, per share		2.769
Series B Convertible Preferred Stock | IPO
Temporary Equity [Line Items]
Amount of dividends in arrears paid			$4,000,000		$4,300,000

Convertible_Preferred_Stock_Sc

Convertible Preferred Stock - Schedule of Convertible Preferred Stock (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2013
Temporary Equity [Line Items]
Convertible Preferred Stock, Proceeds Net of Issuance Costs	$89,009
Series A Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized		35,377,556
Convertible preferred stock, shares issued		34,349,894
Convertible preferred stock, shares outstanding		34,349,894
Convertible Preferred Stock, Proceeds Net of Issuance Costs		33,623
Convertible Preferred Stock, Aggregate Liquidation Preference		38,063
Series B Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized		27,081,680
Convertible preferred stock, shares issued		27,081,680
Convertible preferred stock, shares outstanding		27,081,680
Convertible Preferred Stock, Proceeds Net of Issuance Costs		73,929
Convertible Preferred Stock, Aggregate Liquidation Preference		76,739
Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized		62,459,236
Convertible preferred stock, shares issued		61,431,574
Convertible preferred stock, shares outstanding		61,431,574
Convertible Preferred Stock, Proceeds Net of Issuance Costs		107,552
Convertible Preferred Stock, Aggregate Liquidation Preference		$114,802

StockBased_Awards_Additional_I

Stock-Based Awards - Additional Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 16, 2011	Feb. 05, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total intrinsic value of options exercised	$12,100,000	$1,000,000	$211,000
Weighted-average estimated fair value of stock option	$26.58	$3.82	$0.47
Total estimated fair value of options vested	2,000,000	127,000	172,000
Weighted-average period to recognize cost	3 years 2 months 12 days
Stock-based compensation expense	5,394,000	657,000	891,000
Total unrecognized compensation cost	22,500,000
Founders Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized to be repurchased				2,552,241
Repurchase rights expiration period	2 years
Estimated fair value of restricted shares	1,199,000
Stock-based compensation expense	0	268,000	713,000
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted stock units granted	31,000
Restricted stock units weighted average grant date fair value	$53.23
Total fair value of shares vested	264,000
Total unrecognized compensation expense related to unvested RSUs, net of estimated forfeitures	$1,400,000
Weighted-average period to recognize cost	2 years
Maximum | Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock based awards vesting period	4 years
Minimum | Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock based awards vesting period	2 years
2011 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Minimum percentage of voting rights of all classes of stock	10.00%
Percentage of statutory stock options	110.00%
Option vesting rights, percentage	2.08%
Stock based awards vesting period	4 years
2011 Equity Incentive Plan | First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option vesting rights, percentage	25.00%
2011 Equity Incentive Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options expiration period	10 years
2014 Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share of common stock available for future issuance					2,250,000
Automatic increases in shares available for grant effective date	1-Jan-15
Shares available for grant, ending date	1-Jan-24
2014 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share of common stock available for future issuance					655,038
2014 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of statutory stock options	85.00%
Share of common stock available for future issuance					600,000
Automatic increases in shares available for grant effective date	1-Jan-15
Shares available for grant, ending date	1-Jan-24

StockBased_Awards_Summary_of_O

Stock-Based Awards - Summary of Option Activity Under 2011 Plan and 2014 Plan (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Options, Outstanding, Beginning Balance	2,228,883	1,520,967	1,451,580
Number of Options, Options granted	932,555	1,267,797	499,278
Number of Options, Options exercised	-305,090	-307,366	-420,308
Number of Options, Options cancelled	-116,873	-252,515	-9,583
Number of Options, Outstanding, Ending Balance	2,739,475	2,228,883	1,520,967	1,451,580
Number of Options, Vested and exercisable — December 31, 2014	802,189
Number of Options, Vested and expected to vest — December 31, 2014	2,666,378
Weighted Average Exercise Price, Outstanding, Beginning Balance	$3.41	$0.47	$0.31
Weighted Average Exercise Price, Options granted	$41.12	$5.80	$0.81
Weighted Average Exercise Price, Options exercised	$2.14	$0.55	$0.31
Weighted Average Exercise Price, Options cancelled	$8.92	$1.26	$0.31
Weighted Average Exercise Price, Outstanding, Ending Balance	$16.15	$3.41	$0.47	$0.31
Weighted Average Exercise Price, Vested and exercisable — December 31, 2014	$2.65
Weighted Average Exercise Price, Vested and expected to vest — December 31, 2014	$15.86
Weighted Average Remaining Contractual Term (Years), Outstanding	8 years 5 months 5 days	8 years 10 months 28 days	9 years 1 month 10 days	9 years 10 months 17 days
Weighted Average Remaining Contractual Term (Years), Vested and exercisable — December 31, 2014	7 years 6 months 18 days
Weighted Average Remaining Contractual Term (Years), Vested and expected to vest — December 31, 2014	8 years 4 months 28 days
Aggregate Intrinsic Value, Outstanding	$79,840	$19,468	$2,038
Aggregate Intrinsic Value, Vested and exercisable — December 31, 2014	33,072
Aggregate Intrinsic Value, Vested and expected to vest — December 31, 2014	$78,390

StockBased_Awards_Summary_of_O1

Stock-Based Awards - Summary of Options Outstanding and Exercisable by Exercise Price (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options Outstanding, Numbers Outstanding	2,739,475
Options Outstanding, Weighted-Average Remaining Contractual Term (in Years)	8 years 5 months 5 days
Options Exercisable, Number Exercisable	802,189
Options Exercisable, Weighted- Average Exercise Price	$2.65
Options Exercisable, Weighted-Average Remaining Contractual Term (in Years)	7 years 6 months 18 days
Exercise Price $0.31
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Exercise Price	$0.31
Options Outstanding, Numbers Outstanding	667,554
Options Outstanding, Weighted-Average Remaining Contractual Term (in Years)	6 years 10 months 17 days
Options Exercisable, Number Exercisable	472,481
Options Exercisable, Weighted- Average Exercise Price	$0.31
Options Exercisable, Weighted-Average Remaining Contractual Term (in Years)	6 years 10 months 17 days
Exercise Price $0.81 to $1.82
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Exercise Price, Lower Range	$0.81
Exercise Price, Upper Range	$1.82
Options Outstanding, Numbers Outstanding	453,552
Options Outstanding, Weighted-Average Remaining Contractual Term (in Years)	7 years 11 months 23 days
Options Exercisable, Number Exercisable	136,284
Options Exercisable, Weighted- Average Exercise Price	$1.33
Options Exercisable, Weighted-Average Remaining Contractual Term (in Years)	7 years 11 months 23 days
Exercise Price $4.07 to $6.86
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Exercise Price, Lower Range	$4.07
Exercise Price, Upper Range	$6.86
Options Outstanding, Numbers Outstanding	523,169
Options Outstanding, Weighted-Average Remaining Contractual Term (in Years)	8 years 9 months 22 days
Options Exercisable, Number Exercisable	125,181
Options Exercisable, Weighted- Average Exercise Price	$6.59
Options Exercisable, Weighted-Average Remaining Contractual Term (in Years)	8 years 9 months 22 days
Exercise Price $11.19 to $21.00
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Exercise Price, Lower Range	$11.19
Exercise Price, Upper Range	$21
Options Outstanding, Numbers Outstanding	315,950
Options Outstanding, Weighted-Average Remaining Contractual Term (in Years)	9 years 4 days
Options Exercisable, Number Exercisable	68,243
Options Exercisable, Weighted- Average Exercise Price	$14.26
Options Exercisable, Weighted-Average Remaining Contractual Term (in Years)	9 years 4 days
Exercise Price $33.55 to $34.93
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Exercise Price, Lower Range	$33.55
Exercise Price, Upper Range	$34.93
Options Outstanding, Numbers Outstanding	249,500
Options Outstanding, Weighted-Average Remaining Contractual Term (in Years)	9 years 4 months 24 days
Exercise Price $40.37 to $49.15
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Exercise Price, Lower Range	$40.37
Exercise Price, Upper Range	$49.15
Options Outstanding, Numbers Outstanding	203,750
Options Outstanding, Weighted-Average Remaining Contractual Term (in Years)	9 years 9 months 18 days
Exercise Price $53.03 to $58.02
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Exercise Price, Lower Range	$53.03
Exercise Price, Upper Range	$58.02
Options Outstanding, Numbers Outstanding	326,000
Options Outstanding, Weighted-Average Remaining Contractual Term (in Years)	9 years 5 months 5 days

StockBased_Awards_Summary_of_S

Stock-Based Awards - Summary of Stock-Based Compensation Recognized (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$5,394	$657	$891
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	4,116	293	130
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$1,278	$364	$761

StockBased_Awards_Fair_Value_o

Stock-Based Awards - Fair Value of Stock Option Awards Estimated Using Black-Scholes Option-Pricing Model (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	6 years 2 months 23 days	6 years 3 months	6 years 3 months
Expected volatility	70.00%	74.00%	67.00%
Risk-free interest rate	1.90%		0.60%
Expected dividend rate	0.00%	0.00%	0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate		0.70%
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate		1.60%

Income_Taxes_Effective_Tax_Rat

Income Taxes - Effective Tax Rate of Provision for Income Taxes from Federal Statutory Rate (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
Federal statutory income tax rate	34.00%	34.00%	34.00%
State income taxes, net of federal benefit	4.60%	5.00%	6.00%
Federal tax credits	6.70%	8.20%	8.60%
Nondeductible permanent items	-1.20%	-2.90%
Stock compensation	-1.40%	-0.60%	-1.90%
Change in valuation allowance	-42.70%	-44.60%	-46.00%
Uncertain tax positions		0.90%	-0.70%

Income_Taxes_Schedule_of_Tax_E

Income Taxes - Schedule of Tax Effect of Temporary Differences to Significant Portions of Deferred Tax Assets (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Deferred tax assets:
Net operating loss carryforwards	$32,500	$16,352
Tax credits	16,737	8,692
Stock options	1,058
Accruals and reserves	476	591
Fixed assets and intangibles	455	65
Other	113
Total deferred tax assets	51,339	25,700
Valuation allowance	-51,339	-25,700
Net deferred tax assets	$0	$0

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Tax Credit Carryforward [Line Items]
Deferred tax assets for net operating losses removed from the deferred tax asset schedule and a corresponding decrease to the valuation allowance	$725,000
Valuation allowance increased	25,600,000	15,600,000
Retroactive US Research
Tax Credit Carryforward [Line Items]
Additional research tax credit carryforwards		12,000
Federal
Tax Credit Carryforward [Line Items]
Deferred tax assets operating loss carryforwards subject to expiration	89,000,000	39,400,000
Operating loss carryforwards expiration year	2030
Tax credit carryforwards	986,000	464,000
Tax credit carryforwards expiration year	2030
Orphan Drug Credit	21,900,000	11,600,000
State
Tax Credit Carryforward [Line Items]
Deferred tax assets operating loss carryforwards subject to expiration	90,100,000	50,800,000
Operating loss carryforwards expiration year	2030
Tax credit carryforwards	$1,600,000	$586,000

Income_Taxes_Unrecognized_Tax_

Income Taxes - Unrecognized Tax Benefits (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Balance at beginning of year	$3,725	$1,551	$324
Additions based on tax positions related to current year	3,550	2,169	1,227
Additions (reductions) for tax positions of prior years		5
Balance at end of year	$7,275	$3,725	$1,551

Commitments_and_Contingencies_1

Commitments and Contingencies - Additional Information (Details) (USD $)	0 Months Ended	1 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	1-May-14	Feb. 28, 2014	Jul. 31, 2011	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure Commitments And Contingencies Additional Information Details [Line Items]
Tenant improvement allowance		$369	$376
Lease term		5 years
Rent expense				572	285	265
Reduction in letter of credit on successful payment of lease amount	119
Restricted cash in connection with lease agreement				$594
Minimum
Disclosure Commitments And Contingencies Additional Information Details [Line Items]
Increase in percentage of monthly lease payments			3.00%
Maximum
Disclosure Commitments And Contingencies Additional Information Details [Line Items]
Increase in percentage of monthly lease payments			6.00%

Commitments_and_Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Lease Payments under Non-Cancellable Operating Leases Arrangements (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Commitments And Contingencies Disclosure [Abstract]
2015	$1,173
2016	1,305
2017	1,335
2018	1,366
2019	560
Total	$5,739

Net_Loss_per_Share_Attributabl2

Net Loss per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss per Share (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator:
Net loss	($16,738)	($15,849)	($13,585)	($13,630)	($11,317)	($8,428)	($8,590)	($6,735)	($59,802)	($35,070)	($16,334)
Accretion and dividends on convertible preferred stock									-4,808	-15,219	-3,227
Net loss attributable to common stockholders	($16,738)	($15,849)	($13,585)	($18,438)	($18,665)	($12,590)	($10,829)	($8,205)	($64,610)	($50,289)	($19,561)
Denominator:
Weighted-average common shares outstanding									28,755,758	3,583,522	3,118,798
Less: weighted-average unvested common shares subject to repurchase										-201,033	-1,741,591
Weighted-average shares used to compute net loss per share attributable to common stockholders, basic and diluted									28,755,758	3,382,489	1,377,207
Net loss per share attributable to common stockholders, basic and diluted	($0.52)	($0.50)	($0.45)	($0.85)	($4.98)	($3.48)	($3.32)	($2.84)	($2.25)	($14.87)	($14.20)

Net_Loss_per_Share_Attributabl3

Net Loss per Share Attributable to Common Stockholders - Outstanding Shares of Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	4,540,995	21,819,014	12,013,039
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	1,610,834	19,598,486	8,354,772
Stock options to purchase common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	2,572,729	1,666,036	1,563,217
Unvested restricted stock units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	9,715
Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share		201,033	1,741,591
Convertible Preferred Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	29,051	353,459	353,459
Common Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	318,666

Subsequent_Events_Additional_I

Subsequent Events - Additional Information (Details) (USD $)	0 Months Ended	12 Months Ended	0 Months Ended				1 Months Ended
	1-May-14	Dec. 31, 2014	Apr. 30, 2015	1-May-15	Mar. 09, 2015	Feb. 05, 2014	Feb. 28, 2015	Jul. 31, 2014	Jan. 30, 2014
Subsequent Event [Line Items]
Net proceeds from issuance of common stock		$121,711,000
Offering expenses								400,000
Reduction in letter of credit on successful payment of lease amount	119,000
Lease Amendment | Scenario Forecast
Subsequent Event [Line Items]
Term of entire lease				5 years
Lease termination date				30-Apr-19
Reduction in letter of credit			-119,000
Subsequent Event
Subsequent Event [Line Items]
Offering expenses							600,000
Subsequent Event | Lease Amendment
Subsequent Event [Line Items]
Reduction in letter of credit on successful payment of lease amount					191,000
Subsequent Event | Lease Amendment | Maximum
Subsequent Event [Line Items]
Tenant Improvement allowance					287,000
Subsequent Event | Lease Amendment | Letter of credit
Subsequent Event [Line Items]
Reduction in letter of credit					287,000
Line of Credit, Current					762,000
IPO
Subsequent Event [Line Items]
Common stock issued in initial public offering						6,624,423
Public offering price of the shares sold									$21
Net proceeds from issuance of common stock						129,400,000
Underwriting discounts and commissions						9,700,000
Offering expenses						3,300,000
Net proceeds from initial public offering						121,700,000
IPO | Subsequent Event
Subsequent Event [Line Items]
Common stock issued in initial public offering							3,450,000
Public offering price of the shares sold							$54
Net proceeds from issuance of common stock							175,100,000
Underwriting discounts and commissions							11,200,000
Offering expenses							600,000
Net proceeds from initial public offering							$174,500,000
IPO | Subsequent Event | Underwriters
Subsequent Event [Line Items]
Common stock issued in initial public offering							450,000

Quarterly_Financial_Data_unaud2

Quarterly Financial Data (unaudited) - Summary of Unaudited Quarterly Financial Information (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Information [Abstract]
Operating expenses	$16,943	$15,835	$13,661	$10,339	$9,525	$7,761	$8,247	$6,747	$56,778	$32,280	$15,985
Net loss	-16,738	-15,849	-13,585	-13,630	-11,317	-8,428	-8,590	-6,735	-59,802	-35,070	-16,334
Net loss attributable to common stockholders	($16,738)	($15,849)	($13,585)	($18,438)	($18,665)	($12,590)	($10,829)	($8,205)	($64,610)	($50,289)	($19,561)
Net loss per share attributable to common stockholders, basic and diluted	($0.52)	($0.50)	($0.45)	($0.85)	($4.98)	($3.48)	($3.32)	($2.84)	($2.25)	($14.87)	($14.20)

Quarterly_Financial_Data_unaud3

Quarterly Financial Data (unaudited) - Additional Information (Details)	0 Months Ended
	Jan. 17, 2014
Quarterly Financial Data [Abstract]
Reverse stock split ratio on preferred and common stock	3.1345

Quarterly_Financial_Data_unaud4

Quarterly Financial Data (unaudited) -Revised Statement of Cash Flows (Details) (USD $)	3 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2014	Jun. 30, 2014	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure - Quarterly Financial Data (unaudited) -Revised Statement of Cash Flows (Details) [Line Items]
Net cash used in operating activities	($8,336)	($18,865)	($29,895)	($44,634)	($31,200)	($12,504)
Net cash provided by financing activities	121,754	121,764	182,274	184,971	171	89,240
Scenario, Adjustment
Disclosure - Quarterly Financial Data (unaudited) -Revised Statement of Cash Flows (Details) [Line Items]
Net cash used in operating activities	-10,621	-21,150	-32,180
Net cash provided by financing activities	$124,039	$124,049	$184,559