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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2015	Nov. 04, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q3
Trading Symbol	RARE
Entity Registrant Name	Ultragenyx Pharmaceutical Inc.
Entity Central Index Key	0001515673
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		38,834,703

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 166,686	$ 24,324
Short-term investments	281,697	163,163
Prepaid expenses and other current assets	12,324	5,929
Total current assets	460,707	193,416
Property and equipment, net	6,285	3,033
Restricted cash	912	744
Long-term investments	133,492
Other assets	734	774
Total assets	602,130	197,967
Current liabilities:
Accounts payable	4,422	4,857
Accrued liabilities	20,148	7,575
Deferred rent—current portion	163	85
Total current liabilities	24,733	12,517
Other liabilities	634	505
Total liabilities	25,367	13,022
Commitments and contingencies (Note 9)
Stockholders’ equity:
Preferred stock, par value of $0.001 per share—25,000,000 shares authorized; nil outstanding as of September 30, 2015 and December 31, 2014
Common stock, par value of $0.001 per share—250,000,000 shares authorized; 38,822,177 and 31,934,682 shares issued and outstanding as of September 30, 2015 and December 31, 2014	39	32
Additional paid-in capital	806,202	324,128
Accumulated other comprehensive loss	(39)	(174)
Accumulated deficit	(229,439)	(139,041)
Total stockholders’ equity	576,763	184,945
Total liabilities and stockholders’ equity	$ 602,130	$ 197,967

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Sep. 30, 2015	Dec. 31, 2014
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	25,000,000	25,000,000
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	250,000,000	250,000,000
Common stock, shares issued	38,822,177	31,934,682
Common stock, shares outstanding	38,822,177	31,934,682

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Operating expenses:
Research and development	$ 29,704	$ 12,854	$ 70,172	$ 32,446
General and administrative	10,232	2,981	21,408	7,389
Total operating expenses	39,936	15,835	91,580	39,835
Loss from operations	(39,936)	(15,835)	(91,580)	(39,835)
Other income (expense), net:
Interest income	673	171	1,402	413
Other expense, net	31	(185)	(220)	(3,642)
Total other income (expense), net	704	(14)	1,182	(3,229)
Net loss	(39,232)	(15,849)	(90,398)	(43,064)
Net loss attributable to common stockholders	$ (39,232)	$ (15,849)	$ (90,398)	$ (47,872)
Net loss per share attributable to common stockholders, basic and diluted	$ (1.03)	$ (0.50)	$ (2.51)	$ (1.73)
Shares used in computing net loss per share attributable to common stockholders, basic and diluted	38,268,632	31,631,385	36,086,598	27,697,137

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (39,232)	$ (15,849)	$ (90,398)	$ (43,064)
Other comprehensive income:
Unrealized gain (loss) on available-for-sale securities	170	(70)	135	(129)
Total comprehensive loss	$ (39,062)	$ (15,919)	$ (90,263)	$ (43,193)

- Definition

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

+ References + Details

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Operating activities:
Net loss	$ (90,398)	$ (43,064)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	904	472
Amortization of premium (discount) on investment securities, net	3,700	2,444
Stock-based compensation	15,395	3,393
Revaluation of convertible preferred stock warrant liability		3,324
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(6,395)	(3,699)
Other assets	40	(462)
Accounts payable	(544)	2,693
Accrued liabilities and other liabilities	11,765	2,719
Net cash used in operating activities	(65,533)	(32,180)
Investing activities:
Purchase of property and equipment	(3,032)	(1,696)
Purchase of investments	(477,321)	(160,942)
Proceeds from the sale of investments	63,310	3,003
Proceeds from maturities of investments	158,420	53,509
Increase in restricted cash	(168)	(293)
Net cash used in investing activities	(258,791)	(106,419)
Financing activities:
Proceeds from issuance of common stock, net	466,686	188,905
Payment of preferred stock dividend		(4,346)
Net cash provided by financing activities	466,686	184,559
Net increase in cash and cash equivalents	142,362	45,960
Cash and cash equivalents at beginning of period	24,324	7,427
Cash and cash equivalents at end of period	166,686	53,387
Supplemental disclosures of non-cash investing and financing information:
Costs of fixed assets included in accounts payable and accrued liabilities	$ 1,124	72
Reclassification of warrant liability to equity upon conversion to common stock warrants		6,743
Conversion of Series A and Series B preferred stock to common stock		$ 129,360

Organization

9 Months Ended

Sep. 30, 2015

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization

1.	Organization

Ultragenyx Pharmaceutical Inc. (the Company) is a biopharmaceutical company 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 3 study of aceneuramic acid extended-release (Ace-ER) in patients with GNE myopathy (GNEM), which is also known as 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 UX007 in patients with glucose transporter type-1 deficiency syndrome (Glut1 DS), a brain energy deficiency; a Phase 2 clinical study of UX007 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 diseases that impair bone mineralization. The Company operates in the United States of America and has one reportable segment.

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 offering expenses of $0.6 million, net proceeds were $174.5 million.

In July 2015, the Company completed an underwritten public offering in which the Company sold 2,530,000 shares of common stock, which included 330,000 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 $120.00 per share. The total proceeds that the Company received from the offering were approximately $286.9 million, net of underwriting discounts and commissions of approximately $16.7 million. After deducting offering expenses payable of approximately $0.2 million, net proceeds were $286.7 million.

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2015

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

Basis of Presentation

The accompanying unaudited condensed consolidated financial statements include the amounts of the Company and our wholly-owned subsidiaries and have been prepared in accordance with U.S. general accepted accounting principles (“U.S. GAAP”) for interim financial information and in accordance with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by generally accepted accounting principles for complete financial statements. The unaudited interim consolidated financial statements have been prepared on the same basis as the annual financial statements. In the opinion of management, the accompanying unaudited condensed consolidated financial statements reflect all adjustments (consisting only of normal recurring adjustments) considered necessary for a fair presentation. These financial statements should be read in conjunction with the audited financial statements and notes thereto for the preceding fiscal year contained in the Company’s Annual Report on Form 10-K filed on March 27, 2015 with the SEC.

The results of operations for the three and nine months ended September 30, 2015 are not necessarily indicative of the results to be expected for the year ending December 31, 2015. The condensed balance sheet as of December 31, 2014 has been derived from audited financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements.

Use of Estimates

The preparation of condensed consolidated financial statements in conformity with U.S. 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 study and manufacturing 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.

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. As of September 30, 2015, as a result of the Company’s offering in July 2015, investments with a maturity of one year or less from the balance sheet date are reported as short-term investments and investments with a maturity of greater than one year from the balance sheet date are reported as long-term investments. Unrealized gains and losses are excluded from earnings and were 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 investments 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 investments. The Company’s cash, cash equivalents, and 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, corporate bond issuers and other financial instruments to the extent recorded in the balance sheets.

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 paid 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. In periods when we have incurred a net loss, convertible preferred stock, options and warrants to purchase common stock and convertible preferred stock warrants are considered common stock equivalents, but have been excluded from the calculation of diluted net loss per share attributable to common stockholders, as their effect is antidilutive.

Recently Issued Accounting Pronouncements

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

9 Months Ended

Sep. 30, 2015

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 condensed consolidated 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 following tables set 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):

	September 30, 2015
	Level 1		Level 2		Level 3		Total
Financial Assets:
Money market funds	$	149,082	$	—	$	—	$	149,082
Corporate bonds		—		360,073		—		360,073
Asset backed securities		—		29,784		—		29,784
U.S. Government agency securities		—		23,178		—		23,178
Commercial paper		—		7,887		—		7,887
Total financial assets	$	149,082	$	420,922	$	—	$	570,004

	December 31, 2014
	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

In January 2014, the Company recorded a liability in connection with a convertible preferred stock warrant liability that was classified as a Level 3 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 seven years, risk-free interest rate of 2.19% and expected volatility of 70%. 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. 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):

	Nine Months Ended September 30,
	2014
Fair value, beginning of period	$	3,419
Change in fair value recorded as a loss in other expense, net		3,324
Reclassification of warrant liability to additional paid-in capital		(6,743	)
Fair value, end of period	$	—

The Company recorded $3.3 million in other expense for the nine months ended September 30, 2014, representing the change in fair value of the warrants for the period. There was no corresponding expense during the nine months ended September 30, 2015 as the preferred stock warrants were converted to common stock warrants upon the completion of the IPO and are no longer subject to remeasurement.

- Definition

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

+ References + Details

Balance Sheet Components

9 Months Ended

Sep. 30, 2015

Investments Debt And Equity Securities [Abstract]

Balance Sheet Components

4.Balance Sheet Components

Cash Equivalents and Investments

The fair values of cash equivalents, short-term investments, and long-term investments classified as available-for-sale securities, consisted of the following (in thousands):

	September 30, 2015
			Gross Unrealized
	Amortized Cost		Gains		Losses			Estimated Fair Value
Money market funds classified as cash equivalents	$	149,082	$	—	$	—		$	149,082
Corporate bonds classified as cash equivalents		5,733		—		—			5,733
Commercial paper classified as short-term investments		7,887		—		—			7,887
Corporate bonds classified as short-term investments		265,914		29		(145	)		265,798
Asset backed securities classified as short-term investments		8,010		2		—			8,012
Corporate bonds classified as long-term investments		88,522		71		(51	)		88,542
Asset backed securities classified as long-term investments		21,741		31		—			21,772
U.S. Government agency securities classified as long-term investments		23,154		24		—			23,178
Total	$	570,043	$	157	$	(196	)	$	570,004

	December 31, 2014
			Gross Unrealized
	Amortized Cost		Gains		Losses			Estimated Fair Value
Money market funds classified as cash equivalents	$	8,627	$	—	$	—		$	8,627
Corporate bonds classified as cash equivalents		3,806		1		—			3,807
Corporate bonds classified as short-term investments		149,303		4		(172	)		149,135
Asset backed securities classified as short-term investments		9,546		—		(4	)		9,542
U.S. Government agency securities classified as short-term investments		4,488		1		(4	)		4,485
Other classified as cash equivalents		209		—		—			209
Total	$	175,979	$	6	$	(180	)	$	175,805

At September 30, 2015, the remaining contractual maturities of available-for-sale securities were less than three years. There have been no significant realized gains or losses on available-for-sale securities for the periods presented.

Accrued Liabilities

Accrued liabilities consist of the following (in thousands):

	September 30,		December 31,
	2015		2014
Research and clinical study expenses	$	8,828	$	2,703
Payroll and related expenses		7,668		4,205
Other		3,652		667
Total accrued liabilities	$	20,148	$	7,575

License and Research Agreements

9 Months Ended

Sep. 30, 2015

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), which was amended in August 2015. 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 $0.1 million (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. In addition, the Company is required to make certain payments to Nobelpharma based upon achievement of certain development and approval milestones. The Company has paid $0.5 million in development milestone payments since the inception of the agreement through September 30, 2015. 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 September 30, 2015). The Company will pay a mid-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 a nominal up-front fee, which was recorded as research and development expense in 2010. The Company will be required to make a milestone payment of $0.1 million 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, AAIPharma 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 a nominal up-front fee, 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 $0.3 million 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 a nominal up-front fee, 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 $0.3 million, 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 $0.8 million.

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), which was amended in August 2015. 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 European Union, 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 also be the lead party for development activities conducted in Japan and Korea. 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 and KHK shall be responsible for 100% of the costs for development activities in Japan and Korea. On the applicable transition date in the profit share territory and the European territory, KHK will become the lead party and be responsible for the costs of the development activities. 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 recognized $3.4 million and $1.3 million in expenses for the three months ended September 30, 2015 and 2014, and $7.0 million and $3.3 million in expenses for the nine months ended September 30, 2015 and 2014, respectively for its share of the costs as research and development. As of September 30, 2015 and December 31, 2014, the Company had receivables in the amount of $3.5 million and $1.3 million from KHK, respectively, for this collaboration arrangement.

Common Stock Warrants

9 Months Ended

Sep. 30, 2015

Warrants And Rights Note Disclosure [Abstract]

Common Stock Warrants

6.	Common Stock Warrants

The table sets forth the outstanding common stock warrants for the periods presented:

Outstanding at September 30, 2015		Outstanding at December 31, 2014		Date Issued	Term	Exercise Price
	83,167		83,167	June 2010	10 years	$	3.006
	—		174,651	February 2011	10 years		3.006
	66,533		66,533	June 2011	10 years		3.006
	149,700		324,351

Stock-Based Compensation

9 Months Ended

Sep. 30, 2015

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

7.	Stock-Based Compensation

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 provides for automatic annual increases in shares available for grant, beginning on January 1, 2015 through January 1, 2024. As of September 30, 2015, there were 1,036,380 shares reserved under the 2014 Plan for the future issuance of equity awards. The Company also had 950,295 shares reserved for the 2014 Employee Stock Purchase Plan, for which no shares had been issued.

The table below sets forth the functional classification of stock-based compensation expense, net of estimated forfeitures, for the periods presented (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2015		2014		2015		2014
Research and development	$	5,555	$	1,200	$	10,528	$	2,557
General and administrative		2,341		452		4,867		836
Total stock-based compensation	$	7,896	$	1,652	$	15,395	$	3,393

Defined Contribution Plan

9 Months Ended

Sep. 30, 2015

Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]

Defined Contribution Plan

8.	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. Prior to 2015, the Company had not provided any contributions to the plan. In 2015, the Company began to make contributions to the Plan for eligible participants, and recorded $0.2 million and $0.4 million as contribution expenses for the three and nine months ended September 30, 2015, respectively.

Commitment and Contingencies

9 Months Ended

Sep. 30, 2015

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

9.	Commitments and Contingencies

Commitments

The Company has various manufacturing, clinical, research, and other contracts with vendors in the conduct of the normal course of its business. If a contract with a specific vendor were to be terminated, typically 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 such 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 such claims, and their resolution could be material to the Company for any particular period, depending upon the level of income or loss for the period, as well as the Company’s consolidated balance sheet.

Net Loss per Share Attributable to Common Stockholders

9 Months Ended

Sep. 30, 2015

Earnings Per Share [Abstract]

Net Loss per Share Attributable to Common Stockholders

10.	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 (in thousands, except share and per share data):

	Three Months Ended September 30,						Nine Months Ended September 30,
	2015			2014			2015			2014
Numerator:
Net loss	$	(39,232	)	$	(15,849	)	$	(90,398	)	$	(43,064	)
Accretion and dividends on convertible preferred stock		—			—			—			(4,808	)
Net loss attributable to common stockholders	$	(39,232	)	$	(15,849	)	$	(90,398	)	$	(47,872	)
Denominator:
Weighted-average shares used to compute net loss per share attributable to common stockholders, basic and diluted		38,268,632			31,631,385			36,086,598			27,697,137
Net loss per share attributable to common stockholders, basic and diluted	$	(1.03	)	$	(0.50	)	$	(2.51	)	$	(1.73	)

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:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2015		2014		2015		2014
Convertible preferred stock (as if converted)		—		—		—		2,153,680
Stock options to purchase common stock		3,525,167		2,749,971		3,066,088		2,542,224
Unvested restricted stock units		163,613		11,684		96,827		3,895
Convertible preferred stock warrants (as if converted)		—		—		—		38,842
Common stock warrants		149,700		353,459		211,116		314,617
		3,838,480		3,115,114		3,374,031		5,053,258

Subsequent Events	9 Months Ended
Subsequent Events	Sep. 30, 2015
Subsequent Events [Abstract]
Subsequent Events	11. Subsequent Events Arcturus Research Collaboration and License Agreement In October 2015, the Company entered into a Research Collaboration and License Agreement with Arcturus Therapeutics, Inc. (Arcturus). The Company and Arcturus will collaborate on the research and development of therapies for select rare diseases. As consideration for entering into the arrangement, the Company will pay Arcturus an upfront fee of $10.0 million. Arcturus will have the primary responsibility for conducting certain research services, funded by the Company, and the Company will be responsible for development and commercialization costs. The Company may elect to initiate collaborative development of up to 10 targets and may be required to make additional future payments based on the achievement of development, approval and sales milestones of up to $156.0 million per target plus mid-single to low double-digit percentage royalties on net sales.

Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2015
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements include the amounts of the Company and our wholly-owned subsidiaries and have been prepared in accordance with U.S. general accepted accounting principles (“U.S. GAAP”) for interim financial information and in accordance with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by generally accepted accounting principles for complete financial statements. The unaudited interim consolidated financial statements have been prepared on the same basis as the annual financial statements. In the opinion of management, the accompanying unaudited condensed consolidated financial statements reflect all adjustments (consisting only of normal recurring adjustments) considered necessary for a fair presentation. These financial statements should be read in conjunction with the audited financial statements and notes thereto for the preceding fiscal year contained in the Company’s Annual Report on Form 10-K filed on March 27, 2015 with the SEC. The results of operations for the three and nine months ended September 30, 2015 are not necessarily indicative of the results to be expected for the year ending December 31, 2015. The condensed balance sheet as of December 31, 2014 has been derived from audited financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements.
Use of Estimates	Use of Estimates The preparation of condensed consolidated financial statements in conformity with U.S. 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 study and manufacturing 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.
Investments	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. As of September 30, 2015, as a result of the Company’s offering in July 2015, investments with a maturity of one year or less from the balance sheet date are reported as short-term investments and investments with a maturity of greater than one year from the balance sheet date are reported as long-term investments. Unrealized gains and losses are excluded from earnings and were 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 investments 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 investments. The Company’s cash, cash equivalents, and 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, corporate bond issuers and other financial instruments to the extent recorded in the balance sheets.
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 paid 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. In periods when we have incurred a net loss, convertible preferred stock, options and warrants to purchase common stock and convertible preferred stock warrants are considered common stock equivalents, but have been excluded from the calculation of diluted net loss per share attributable to common stockholders, as their effect is antidilutive.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements 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.

- Definition

Disclosure of accounting policy for cash and cash equivalents, including the policy for determining which items are treated as cash equivalents. Other information that may be disclosed includes (1) the nature of any restrictions on the entity's use of its cash and cash equivalents, (2) whether the entity's cash and cash equivalents are insured or expose the entity to credit risk, (3) the classification of any negative balance accounts (overdrafts), and (4) the carrying basis of cash equivalents (for example, at cost) and whether the carrying amount of cash equivalents approximates fair value.

+ References + Details

- Definition

Disclosure of accounting policy for income taxes, which may include its accounting policies for recognizing and measuring deferred tax assets and liabilities and related valuation allowances, recognizing investment tax credits, operating loss carryforwards, tax credit carryforwards, and other carryforwards, methodologies for determining its effective income tax rate and the characterization of interest and penalties in the financial statements.

+ References + Details

- Definition

Disclosure of accounting policy for investments in financial assets, including marketable securities (debt and equity securities with readily determinable fair values), investments accounted for under the equity method and cost method, securities borrowed and loaned, and repurchase and resale agreements. For marketable securities, the disclosure may include the entity's accounting treatment for transfers between investment categories and how the fair values for such securities are determined. Also, for all investments, an entity may describe its policy for assessing, recognizing and measuring impairment of the investment.

+ References + Details

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2015

Fair Value Disclosures [Abstract]

Summary of Financial Assets Measured on Recurring Basis

The following tables set 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):

	September 30, 2015
	Level 1		Level 2		Level 3		Total
Financial Assets:
Money market funds	$	149,082	$	—	$	—	$	149,082
Corporate bonds		—		360,073		—		360,073
Asset backed securities		—		29,784		—		29,784
U.S. Government agency securities		—		23,178		—		23,178
Commercial paper		—		7,887		—		7,887
Total financial assets	$	149,082	$	420,922	$	—	$	570,004

	December 31, 2014
	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

Summary of Changes in Estimated Fair Value

	Nine Months Ended September 30,
	2014
Fair value, beginning of period	$	3,419
Change in fair value recorded as a loss in other expense, net		3,324
Reclassification of warrant liability to additional paid-in capital		(6,743	)
Fair value, end of period	$	—

Balance Sheet Components (Tables)

9 Months Ended

Sep. 30, 2015

Investments Debt And Equity Securities [Abstract]

Summary of Fair Values of Cash Equivalents, Short-Term Investments and Long-Term Investments Classified as Available-for-Sale Securities

The fair values of cash equivalents, short-term investments, and long-term investments classified as available-for-sale securities, consisted of the following (in thousands):

	September 30, 2015
			Gross Unrealized
	Amortized Cost		Gains		Losses			Estimated Fair Value
Money market funds classified as cash equivalents	$	149,082	$	—	$	—		$	149,082
Corporate bonds classified as cash equivalents		5,733		—		—			5,733
Commercial paper classified as short-term investments		7,887		—		—			7,887
Corporate bonds classified as short-term investments		265,914		29		(145	)		265,798
Asset backed securities classified as short-term investments		8,010		2		—			8,012
Corporate bonds classified as long-term investments		88,522		71		(51	)		88,542
Asset backed securities classified as long-term investments		21,741		31		—			21,772
U.S. Government agency securities classified as long-term investments		23,154		24		—			23,178
Total	$	570,043	$	157	$	(196	)	$	570,004

	December 31, 2014
			Gross Unrealized
	Amortized Cost		Gains		Losses			Estimated Fair Value
Money market funds classified as cash equivalents	$	8,627	$	—	$	—		$	8,627
Corporate bonds classified as cash equivalents		3,806		1		—			3,807
Corporate bonds classified as short-term investments		149,303		4		(172	)		149,135
Asset backed securities classified as short-term investments		9,546		—		(4	)		9,542
U.S. Government agency securities classified as short-term investments		4,488		1		(4	)		4,485
Other classified as cash equivalents		209		—		—			209
Total	$	175,979	$	6	$	(180	)	$	175,805

Accrued Liabilities

Accrued liabilities consist of the following (in thousands):

	September 30,		December 31,
	2015		2014
Research and clinical study expenses	$	8,828	$	2,703
Payroll and related expenses		7,668		4,205
Other		3,652		667
Total accrued liabilities	$	20,148	$	7,575

Common Stock Warrants (Tables)

9 Months Ended

Sep. 30, 2015

Warrants And Rights Note Disclosure [Abstract]

Summary of Outstanding Warrants

The table sets forth the outstanding common stock warrants for the periods presented:

Outstanding at September 30, 2015		Outstanding at December 31, 2014		Date Issued	Term	Exercise Price
	83,167		83,167	June 2010	10 years	$	3.006
	—		174,651	February 2011	10 years		3.006
	66,533		66,533	June 2011	10 years		3.006
	149,700		324,351

- Definition

Tabular disclosure of warrants or rights issued. Warrants and rights outstanding are derivative securities that give the holder the right to purchase securities (usually equity) from the issuer at a specific price within a certain time frame. Warrants are often included in a new debt issue to entice investors by a higher return potential. The main difference between warrants and call options is that warrants are issued and guaranteed by the company, whereas options are exchange instruments and are not issued by the company. Also, the lifetime of a warrant is often measured in years, while the lifetime of a typical option is measured in months. Disclose the title of issue of securities called for by warrants and rights outstanding, the aggregate amount of securities called for by warrants and rights outstanding, the date from which the warrants or rights are exercisable, and the price at which the warrant or right is exercisable.

+ References + Details

Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2015

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Functional Classification of Stock-Based Compensation Expense, Net of Estimated Forfeitures

The table below sets forth the functional classification of stock-based compensation expense, net of estimated forfeitures, for the periods presented (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2015		2014		2015		2014
Research and development	$	5,555	$	1,200	$	10,528	$	2,557
General and administrative		2,341		452		4,867		836
Total stock-based compensation	$	7,896	$	1,652	$	15,395	$	3,393

Net Loss per Share Attributable to Common Stockholders (Tables)

9 Months Ended

Sep. 30, 2015

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 (in thousands, except share and per share data):

	Three Months Ended September 30,						Nine Months Ended September 30,
	2015			2014			2015			2014
Numerator:
Net loss	$	(39,232	)	$	(15,849	)	$	(90,398	)	$	(43,064	)
Accretion and dividends on convertible preferred stock		—			—			—			(4,808	)
Net loss attributable to common stockholders	$	(39,232	)	$	(15,849	)	$	(90,398	)	$	(47,872	)
Denominator:
Weighted-average shares used to compute net loss per share attributable to common stockholders, basic and diluted		38,268,632			31,631,385			36,086,598			27,697,137
Net loss per share attributable to common stockholders, basic and diluted	$	(1.03	)	$	(0.50	)	$	(2.51	)	$	(1.73	)

Outstanding Shares of Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share

	Three Months Ended September 30,				Nine Months Ended September 30,
	2015		2014		2015		2014
Convertible preferred stock (as if converted)		—		—		—		2,153,680
Stock options to purchase common stock		3,525,167		2,749,971		3,066,088		2,542,224
Unvested restricted stock units		163,613		11,684		96,827		3,895
Convertible preferred stock warrants (as if converted)		—		—		—		38,842
Common stock warrants		149,700		353,459		211,116		314,617
		3,838,480		3,115,114		3,374,031		5,053,258

Organization - Additional Information (Details) $ / shares in Units, $ in Millions	1 Months Ended		9 Months Ended
	Jul. 31, 2015 USD ($) $ / shares shares	Feb. 28, 2015 USD ($) $ / shares shares	Sep. 30, 2015 Segment shares	Dec. 31, 2014 shares
Organization And Nature Of Business [Line Items]
Common stock, shares issued \| shares	2,530,000	3,450,000	38,822,177	31,934,682
Purchase of additional shares by underwriters \| shares	330,000	450,000
Common stock issued price per share \| $ / shares	$ 120.00	$ 54.00
Proceeds from public offering before offering expenses	$ 286.9	$ 175.1
Underwriting discounts and commissions	16.7	11.2
Offering expenses	0.2	0.6
Proceeds from public offering, net	$ 286.7	$ 174.5
United States of America
Organization And Nature Of Business [Line Items]
Number of reportable segments \| Segment			1

Summary of Significant Accounting Policies - Additional Information (Details)	9 Months Ended
	Sep. 30, 2015 USD ($)
Accounting Policies [Abstract]
Unrecognized tax benefits related to interest or penalties	$ 0

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financial Assets:
Fair value	$ 570,004	$ 175,805
Money Market Funds
Financial Assets:
Fair value	149,082	8,627
Asset Backed Securities
Financial Assets:
Fair value	29,784	9,542
Other
Financial Assets:
Fair value		209
Level 1
Financial Assets:
Fair value	149,082	8,627
Level 1 \| Money Market Funds
Financial Assets:
Fair value	149,082	8,627
Level 2
Financial Assets:
Fair value	420,922	167,178
Level 2 \| Asset Backed Securities
Financial Assets:
Fair value	29,784	9,542
Level 2 \| Other
Financial Assets:
Fair value		209
Corporate Bonds
Financial Assets:
Fair value	360,073	152,942
Corporate Bonds \| Level 2
Financial Assets:
Fair value	360,073	152,942
US Government Corporations and Agencies Securities
Financial Assets:
Fair value	23,178	4,485
US Government Corporations and Agencies Securities \| Level 2
Financial Assets:
Fair value	23,178	$ 4,485
Commercial Paper
Financial Assets:
Fair value	7,887
Commercial Paper \| Level 2
Financial Assets:
Fair value	$ 7,887

Fair Value Measurements - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		9 Months Ended
	Jan. 30, 2014	Sep. 30, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Revaluation of convertible preferred stock warrant liability		$ 3,324
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 upon IPO closing, per share.	$ 21.00
Expected time	7 years
Expected risk free interest rate	2.19%
Expected volatility	70.00%

Fair Value Measurements - Summary of Changes in Estimated Fair Value (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2014 USD ($)
Fair Value Disclosures [Abstract]
Fair value, beginning of period	$ 3,419
Change in fair value recorded as a loss in other expense, net	3,324
Reclassification of warrant liability to additional paid-in capital	$ (6,743)

Balance Sheet Components - Summary of Fair Values of Cash Equivalents, Short-Term Investments and Long-Term Investments Classified as Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 570,043	$ 175,979
Gross Unrealized Gains	157	6
Gross Unrealized Losses	(196)	(180)
Estimated Fair Value	570,004	175,805
Money Market Funds Classified as Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	149,082	8,627
Estimated Fair Value	149,082	8,627
Corporate Bonds Classified as Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	5,733	3,806
Gross Unrealized Gains		1
Estimated Fair Value	5,733	3,807
Other Classified as Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		209
Estimated Fair Value		209
Commercial Paper Classified as Short-Term Investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	7,887
Estimated Fair Value	7,887
Corporate Bonds Classified as Short-Term Investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	265,914	149,303
Gross Unrealized Gains	29	4
Gross Unrealized Losses	(145)	(172)
Estimated Fair Value	265,798	149,135
Asset Backed Securities Classified as Short-Term Investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	8,010	9,546
Gross Unrealized Gains	2
Gross Unrealized Losses		(4)
Estimated Fair Value	8,012	9,542
Corporate Bonds Classified as Long-Term Investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	88,522
Gross Unrealized Gains	71
Gross Unrealized Losses	(51)
Estimated Fair Value	88,542
Asset Backed Securities Classified as Long-Term Investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	21,741
Gross Unrealized Gains	31
Estimated Fair Value	21,772
U.S. Government Agency Securities Classified as Long-Term Investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	23,154
Gross Unrealized Gains	24
Estimated Fair Value	$ 23,178
U.S. Government Agency Securities Classified as Short-Term Investments
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 - Additional Information (Details)	9 Months Ended
	Sep. 30, 2015 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Significant realized gains or losses on available-for-sale securities	$ 0
Maximum
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities remaining contractual maturities	3 years

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Payables And Accruals [Abstract]
Research and clinical study expenses	$ 8,828	$ 2,703
Payroll and related expenses	7,668	4,205
Other	3,652	667
Total accrued liabilities	$ 20,148	$ 7,575

License and Research Agreements - Additional Information (Details) $ in Thousands, ¥ in Millions	1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended	61 Months Ended
	Jun. 30, 2013 USD ($)	Sep. 30, 2010 USD ($) shares	Sep. 30, 2010 JPY (¥) shares	Sep. 30, 2015 USD ($) shares	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) shares	Sep. 30, 2014 USD ($)	Dec. 31, 2012 USD ($)	Sep. 30, 2015 USD ($) shares	Sep. 30, 2015 JPY (¥) shares	Jul. 31, 2015 shares	Feb. 28, 2015 shares	Dec. 31, 2014 USD ($) shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development				$ 29,704	$ 12,854	$ 70,172	$ 32,446
Common stock, shares issued \| shares				38,822,177		38,822,177			38,822,177	38,822,177	2,530,000	3,450,000	31,934,682
Accrued liabilities				$ 20,148		$ 20,148			$ 20,148				$ 7,575
Nobelpharma License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development		$ 100	¥ 10						500
Common stock, shares issued \| shares		76,567	76,567
Remaining total aggregate payments				1,700		1,700			1,700	¥ 200
Saint Louis University License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments				100		$ 100			100
H I B M Research Group License Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payments for royalty on net sales						1.00%
H I B M Research Group License Agreement \| Development and Approval Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments				300		$ 300			300
St. Jude Children's Research Hospital License Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payments for royalty on net sales						1.00%
Baylor Research Institute License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development								$ 300
Payment of option exercise fee	$ 800
Baylor Research Institute License Agreement \| Development Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments				10,500		$ 10,500			10,500
Baylor Research Institute License Agreement \| Sales Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments				7,500		7,500			7,500
Kyowa Hakko Kirin Collaboration \| License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research and development				3,400	$ 1,300	7,000	$ 3,300
License agreement receivable				$ 3,500		$ 3,500			$ 3,500				$ 1,300

Common Stock Warrants - Summary of Outstanding Warrants (Details) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Class Of Warrant Or Right [Line Items]
Number of Warrants Outstanding	149,700	324,351
June 2010 Common Stock Warrant [Member]
Class Of Warrant Or Right [Line Items]
Number of Warrants Outstanding	83,167	83,167
Date Issued	2010-06	2010-06
Term	10 years	10 years
Exercise Price	$ 3.006	$ 3.006
February 2011 Common Stock Warrant [Member]
Class Of Warrant Or Right [Line Items]
Number of Warrants Outstanding		174,651
Date Issued	2011-02	2011-02
Term	10 years	10 years
Exercise Price	$ 3.006	$ 3.006
June 2011 Common Stock Warrant [Member]
Class Of Warrant Or Right [Line Items]
Number of Warrants Outstanding	66,533	66,533
Date Issued	2011-06	2011-06
Term	10 years	10 years
Exercise Price	$ 3.006	$ 3.006

Stock-Based Compensation - Additional Information (Details)	9 Months Ended
	Sep. 30, 2015 shares
2014 Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share reserved for future issuance	1,036,380
Automatic increases in shares available for grant effective date	Jan. 01, 2015
Shares available for grant, ending date	Jan. 01, 2024
2014 Employee Stock Purchase plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share reserved for future issuance	950,295
Shares issued	0

Stock-Based Compensation - Functional Classification of Stock-Based Compensation Expense, Net of Estimated Forfeitures (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 7,896	$ 1,652	$ 15,395	$ 3,393
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	5,555	1,200	10,528	2,557
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 2,341	$ 452	$ 4,867	$ 836

Defined Contribution Plan - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Contribution expenses	$ 0.2	$ 0.4

Net Loss per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator:
Net loss	$ (39,232)	$ (15,849)	$ (90,398)	$ (43,064)
Accretion and dividends on convertible preferred stock				(4,808)
Net loss attributable to common stockholders	$ (39,232)	$ (15,849)	$ (90,398)	$ (47,872)
Denominator:
Weighted-average shares used to compute net loss per share attributable to common stockholders, basic and diluted	38,268,632	31,631,385	36,086,598	27,697,137
Net loss per share attributable to common stockholders, basic and diluted	$ (1.03)	$ (0.50)	$ (2.51)	$ (1.73)

- Definition

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

+ References + Details

Net Loss per Share Attributable to Common Stockholders - Outstanding Shares of Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	3,838,480	3,115,114	3,374,031	5,053,258
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				2,153,680
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	3,525,167	2,749,971	3,066,088	2,542,224
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	163,613	11,684	96,827	3,895
Convertible Preferred Stock Warrants (as if converted)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share				38,842
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	149,700	353,459	211,116	314,617

Subsequent Events - Additional Information (Details) - Subsequent Event - Arcturus Research Collaboration And License Agreement $ in Millions	Oct. 31, 2015 USD ($) Target
Subsequent Event [Line Items]
Upfront fee payable for research and development	$ 10.0
Future contingent milestone payments	$ 156.0
Maximum
Subsequent Event [Line Items]
Number of collaborative development targets \| Target	10