Aravive (ARAV) 10-K 6 Mar 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Feb. 28, 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	VSAR
Entity Registrant Name	Versartis, Inc.
Entity Central Index Key	1513818
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		29,255,436
Entity Public Float			$182,105,415

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current assets
Cash and cash equivalents	$170,566	$13,288
Prepaid expenses and other current assets	2,398	978
Total current assets	172,964	14,266
Other assets	616	396
Property and equipment, net	714	21
Total assets	174,294	14,683
Current liabilities
Accounts payable	1,259	315
Accrued liabilities	5,666	3,668
Total current liabilities	6,925	3,983
Convertible preferred stock warrant liability		474
Convertible preferred stock call option liability		21
Total liabilities	6,925	4,478
Commitments and contingencies (Note 7)
Convertible preferred stock, $0.0001 par value; 5,000,000, and 135,816,462 shares authorized at December 31, 2014 and December 31, 2013, respectively; zero and 120,648,174 shares issued and outstanding at December 31, 2014 and December 31, 2013, respectively; zero and $60,392 liquidation preference at December 31, 2014 and December 31, 2013, respectively		57,497
Stockholders' equity (deficit)
Common stock, $0.0001 par value, 50,000,000 and 15,652,174 shares authorized at December 31, 2014 and December 31, 2013, respectively; 24,245,437 and 1,257,311 shares issued and outstanding at December 31, 2014 and December 31, 2013, respectively	2
Additional paid-in capital	278,626	6,454
Deficit accumulated during the development stage	-111,259	-53,746
Total stockholders' equity (deficit)	167,369	-47,292
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	$174,294	$14,683

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, except Share data, unless otherwise specified
Statement Of Financial Position [Abstract]
Convertible preferred stock, par value	$0.00	$0.00
Convertible preferred stock, shares authorized	5,000,000	135,816,462
Convertible preferred stock, shares issued	0	120,648,174
Convertible preferred stock, shares outstanding	0	120,648,174
Convertible preferred stock, liquidation preference	$0	$60,392
Common stock, par value	$0.00	$0.00
Common stock, shares authorized	50,000,000	15,652,174
Common stock, shares issued	24,245,437	1,257,311
Common stock, shares outstanding	24,245,437	1,257,311

Consolidated_Statements_of_Ope

Consolidated Statements of Operations and Comprehensive Loss (USD $)	12 Months Ended			73 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Operating expenses
Research and development	$32,608	$14,855	$10,963	$78,481
General and administrative	13,505	4,428	1,936	23,646
Total operating expenses	46,113	19,283	12,899	102,127
Loss from operations	-46,113	-19,283	-12,899	-102,127
Interest income	132	1		135
Interest expense		-128	-393	-863
Other income (expense), net	-11,532	913	75	-9,502
Net loss and comprehensive loss	-57,513	-18,497	-13,217	-112,357
Deemed dividend related to beneficial conversion feature of convertible preferred stock	-25,559			-25,559
Accretion of Series A preferred stock to redemption value, net of extinguishment				1,098
Net loss attributable to common stockholders	($83,072)	($18,497)	($13,217)	($136,818)
Net loss per basic and diluted share attributable to common stockholders	($4.39)	($41.10)	($114.71)
Weighted-average common shares used to compute basic and diluted net loss per share	18,921,533	450,000	115,219

Consolidated_Statements_of_Con

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (USD $)	Total	IPO [Member]	Series A Preferred Stock [Member]	Series B Preferred Stock [Member]	Series B Preferred Stock [Member]	Series E Preferred Stock [Member]	Series C Convertible Preferred Stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Common Stock [Member]	Common Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]	Deficit Accumulated During the Development Stage [Member]	Deficit Accumulated During the Development Stage [Member]
	USD ($)	USD ($)	USD ($)	USD ($)	Issuance Date of January and May 2012 [Member]	USD ($)	USD ($)	USD ($)	IPO [Member]	Series A Preferred Stock [Member]	Series A Preferred Stock [Member]	Series A Preferred Stock [Member]	Series B Preferred Stock [Member]	Series B Preferred Stock [Member]	Series B Preferred Stock [Member]	Series B Preferred Stock [Member]	Series B Preferred Stock [Member]	Series B Preferred Stock [Member]	Series B Preferred Stock [Member]	Series E Preferred Stock [Member]	Series C Convertible Preferred Stock [Member]	Series D Preferred Stock [Member]	Series D-2 Preferred [Member]	USD ($)	IPO [Member]	Series B Preferred Stock [Member]	USD ($)	IPO [Member]	Series B Preferred Stock [Member]	Series B Preferred Stock [Member]	Series E Preferred Stock [Member]	USD ($)	Series A Preferred Stock [Member]
					USD ($)				USD ($)	USD ($)	Issuance Date of December 2008 [Member]	Issuance Date of May and December 2009 and April 2010 [Member]	USD ($)	Issuance Date of February 2011 [Member]	Issuance Date of January 2012 [Member]	Issuance Date of May 2012 [Member]	Issuance Date of January and May 2012 [Member]	Convertible notes [Member]	Convertible notes [Member]	USD ($)	USD ($)	USD ($)	USD ($)		USD ($)			USD ($)	USD ($)	Issuance Date of January and May 2012 [Member]	USD ($)		USD ($)
											USD ($)	USD ($)		USD ($)	USD ($)	USD ($)	USD ($)	USD ($)	Issuance Date of February 2011 [Member]											USD ($)
																			USD ($)
Beginning Balances at Dec. 10, 2008
Issuance of Series A convertible preferred stock valued at $0.0001 per share in consideration for research and development license in December 2008											$1,000
Issuance of Series A convertible preferred stock valued at $0.0001 per share in consideration for research and development license in December 2008, shares											11,000,000
Accretion to redemption value of convertible preferred stock	-10,999,000							10,999,000																								-10,999,000
Issuance of Series A convertible preferred stock for cash at $1.00 per share, net of issuance costs of $3, in May and December 2009 and April 2010												10,997,000
Issuance of Series A convertible preferred stock for cash at $1.00 per share, net of issuance costs of $3, in May and December 2009 and April 2010, shares												11,000,000
Accretion to redemption value of convertible preferred stock- offering costs	-3,000							3,000																								-3,000
Issuance of common stock upon exercise of options subject to repurchase, shares																								127,536
Extinguishment of Series A convertible preferred stock (Refer to Note 8)			12,100,000							-12,100,000																							12,100,000
Issuance of stock														8,215,000
Issuance of stock, shares														21,805,693
Conversion of convertible securities																			4,607,000
Conversion of convertible securities, shares																			10,238,444
Issuance of common stock upon exercise of options	3,000																										3,000
Issuance of common stock upon exercise of options, shares																								2,261
Vesting of options subject to repurchase	11,000																										11,000
Stock-based compensation	192,000																										192,000
Net loss	-23,130,000																															-23,130,000
Ending Balances at Dec. 31, 2011	-21,826,000																										206,000					-22,032,000
Ending Balances at Dec. 31, 2011								22,722,000
Ending Balances, shares at Dec. 31, 2011																								129,797
Ending Balances, shares at Dec. 31, 2011								54,044,137
Issuance of stock					152,000										2,999,000	3,026,000	900,000													152,000
Issuance of stock, shares															6,430,555	6,430,555	2,000,000
Issuance of common stock upon exercise of options	5,000																										5,000
Issuance of common stock upon exercise of options, shares																								3,910
Vesting of options subject to repurchase	3,000																										3,000
Stock-based compensation	141,000																										141,000
Net loss	-13,217,000																															-13,217,000
Ending Balances at Dec. 31, 2012	-34,742,000																										507,000					-35,249,000
Ending Balances at Dec. 31, 2012								29,647,000
Ending Balances, shares at Dec. 31, 2012																								133,707
Ending Balances, shares at Dec. 31, 2012								68,905,247
Issuance of stock, shares																					14,222,222
Ending Balances at Jan. 31, 2013
Beginning Balances at Dec. 31, 2012	-34,742,000																										507,000					-35,249,000
Beginning Balances at Dec. 31, 2012								29,647,000
Beginning Balances, shares at Dec. 31, 2012																								133,707
Beginning Balances, shares at Dec. 31, 2012								68,905,247
Issuance of stock																					19,301,000	9,665,000
Issuance of stock, shares																					36,444,444	17,777,777
Conversion of convertible securities				5,704,000									-5,704,000					4,588,000											5,704,000
Conversion of convertible securities, shares													-12,674,846					10,195,552								1,102,160
Issuance of common stock upon exercise of options	27,000																										27,000
Issuance of common stock upon exercise of options, shares	21,444																							21,444
Vesting of options subject to repurchase	1,000																										1,000
Stock-based compensation	215,000																										215,000
Net loss	-18,497,000																															-18,497,000
Ending Balances at Dec. 31, 2013	-47,292,000																										6,454,000					-53,746,000
Ending Balances at Dec. 31, 2013	57,497,000		9,900,000	18,631,000			19,301,000	57,497,000
Ending Balances, shares at Dec. 31, 2013																								1,257,311
Ending Balances, shares at Dec. 31, 2013	120,648,174		22,000,000	44,425,953			36,444,444	120,648,174
Issuance of stock		132,137,000																										132,137,000
Issuance of stock, shares																									6,900,000
Conversion of convertible securities		122,290,000							-122,290,000																2,000			122,288,000
Conversion of convertible securities, shares									-182,575,322																15,876,104
Issuance of convertible preferred stock																				54,816,000			9,977,000
Issuance of convertible preferred stock, shares																				48,758,857			13,168,291
Reclassification of warrant liability upon closing of IPO		2,752,000																										2,752,000
Reclassification of call option liability upon closing of IPO		9,581,000																										9,581,000
Issuance of common stock upon exercise of warrants	572,000																										572,000
Issuance of common stock upon exercise of warrants, shares																								158,179
Issuance of common stock upon exercise of options	59,000																										59,000
Issuance of common stock upon exercise of options, shares	44,822																							44,822
Issuance of common stock under employee benefit plans	142,000																										142,000
Issuance of common stock under employee benefit plans, shares																								9,021
Beneficial conversion feature related to the issuance of Series E preferred stock						25,559,000														-25,559,000											25,559,000
Deemed dividend related to beneficial conversion feature of convertible preferred stock	-25,559,000					-25,559,000														25,559,000											-25,559,000
Stock-based compensation	4,641,000																										4,641,000
Net loss	-57,513,000																															-57,513,000
Ending Balances at Dec. 31, 2014	$167,369,000																							$2,000			$278,626,000					($111,259,000)
Ending Balances, shares at Dec. 31, 2014																								24,245,437
Ending Balances, shares at Dec. 31, 2014	0

Consolidated_Statements_of_Con1

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) (USD $)	12 Months Ended	37 Months Ended	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2013
Stock, par value per share	$0.00			$0.00
Common Stock [Member] | IPO [Member]
Convertible preferred stock, issuance cost	$2,620
Series A Preferred Stock [Member] | Issuance Date of December 2008 [Member]
Stock, par value per share		$0.00
Series A Preferred Stock [Member] | Issuance Date of May and December 2009 and April 2010 [Member]
Stock, issuance price per share		$1
Convertible preferred stock, issuance cost		3
Series B Preferred Stock [Member] | Issuance Date of February 2011 [Member]
Stock, issuance price per share		$0.45
Convertible preferred stock, issuance cost		210
Series B Preferred Stock [Member] | Issuance Date of February 2011 [Member] | Convertible notes [Member]
Stock, issuance price per share		$0.45
Series B Preferred Stock [Member] | Issuance Date of January 2012 [Member]
Stock, issuance price per share			$0.45
Convertible preferred stock, issuance cost			23
Convertible preferred stock, call option liability			129
Series B Preferred Stock [Member] | Issuance Date of May 2012 [Member]
Stock, issuance price per share			$0.45
Convertible preferred stock, call option liability			132
Series C Convertible Preferred Stock [Member]
Stock, issuance price per share				$0.56
Convertible preferred stock, issuance cost				335
Convertible preferred stock, call option liability				864
Series D Preferred Stock [Member]
Stock, issuance price per share				$0.56
Convertible preferred stock, issuance cost				209
Convertible preferred stock, call option liability				126
Series D-2 Preferred [Member] | Issuance Date of January and May 2012 [Member]
Stock, issuance price per share	$0.76
Convertible preferred stock, issuance cost	23
Series E Preferred Stock [Member] | Issuance Date of January and May 2012 [Member]
Stock, issuance price per share	$1.13
Convertible preferred stock, issuance cost	$184

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended			73 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Cash flows from operating activities
Net loss	($57,513)	($18,497)	($13,217)	($112,357)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	107	17	21	200
Loss on disposition of assets	26			35
Reserve for uncollectible receivables				54
Stock-based compensation expense	4,641	215	141	5,189
Amortization of debt discount		121	312	666
Non-cash interest expense		7	81	195
Non-cash research and development expense				1
Remeasurement of convertible preferred stock call option liability	9,560	-969	-89	7,463
Remeasurement of convertible preferred stock warrant liability	2,279	41		2,239
Changes in assets and liabilities
Accounts receivable		84	491
Prepaid expenses and other assets	-1,695	299	-128	-3,013
Accounts payable	944	-675	413	1,259
Accrued liabilities and other liabilities	1,998	2,267	259	5,665
Net cash used in operating activities	-39,653	-17,090	-11,716	-92,404
Cash flows from investing activities
Proceeds from sale of property and equipment				10
Purchase of property and equipment	-827	-9		-1,012
Security deposit for facility lease	55
Net cash used in investing activities	-772	-9		-1,002
Cash flows from financing activities
Proceeds from issuance of common stock in initial public offering, net of issuance costs	132,137			132,137
Proceeds from sale of option for Series A preferred stock purchase rights				1,000
Proceeds from issuance of convertible preferred stock, net of issuance costs	64,793	29,956	5,765	120,113
Proceeds from exercise of convertible preferred stock warrants	572		900	1,472
Proceeds from issuance of common stock in connection with employee benefit plans	201	27	5	250
Proceeds from issuance of convertible notes payable			4,500	9,000
Net cash provided by financing activities	197,703	29,983	11,170	263,972
Net increase (decrease) in cash and cash equivalents	157,278	12,884	-546	170,566
Cash and cash equivalents at beginning of period	13,288	404	950
Cash and cash equivalents at end of period	170,566	13,288	404	170,566
Supplemental disclosure
Cash paid for interest				2
Supplemental disclosure of noncash items
Conversion of notes payable and accrued interest to preferred stock		4,588		9,195
Issuance of warrants for preferred stock in connection with convertible notes			433	666
Accretion of Series A convertible preferred stock to redemption value				11,002
Issuance of call options related to convertible preferred stock		1,116		3,504
Extinguishment of Series A convertible preferred stock				12,100
Conversion of preferred stock call option liability [Member]
Supplemental disclosure of noncash items
Conversion of liability to additional paid in capital	9,581
Conversion of preferred stock warrant liability [Member]
Supplemental disclosure of noncash items
Conversion of liability to additional paid in capital	2,752
Conversion of preferred stock to common stock [Member]
Supplemental disclosure of noncash items
Conversion of liability to additional paid in capital	$122,290

Formation_and_Business_of_the_

Formation and Business of the Company	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Formation And Business of the Company	1. Formation and Business of the Company
	Versartis, Inc., (the “Company”) a development stage company, was incorporated on December 10, 2008 in the State of Delaware. The Company is an endocrine-focused biopharmaceutical company initially developing long-acting recombinant human growth hormone for the treatment of growth hormone deficiency. The Company is developing drug candidates that it has licensed from Amunix Operating, Inc. (“Amunix”).
	The Company’s headquarters and operations are in Menlo Park, California. Since incorporation, the Company has been primarily performing research and development activities, including early clinical trials, filing patent applications, obtaining regulatory approvals, hiring personnel, and raising capital to support and expand these activities.
	Initial Public Offering
	In March 2014, the Company completed its initial public offering of shares of its common stock, or IPO, pursuant to which the Company issued 6,900,000 shares of common stock, which includes shares issued pursuant to the underwriters’ exercise of their over-allotment option, and received net proceeds of approximately $132.1 million, after underwriting discounts, commissions and offering expenses. In addition, in connection with the completion of the Company’s IPO, all convertible preferred stock converted into common stock. Effective with the closing of the IPO, the Company’s Amended and restated Certificate of Incorporation authorizes the Company to issue 50.0 million shares of common stock and 5.0 million shares of preferred stock.

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
	Basis of Presentation and Use of Estimates
	The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”). The preparation of the accompanying consolidated financial statements in accordance with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of expenses during the reporting period. Actual results could differ from those estimates.
	The accompanying financial statements are consolidated for the year ended December 31, 2014 and include the accounts of Versartis, Inc. and its wholly-owned subsidiary, Versartis Cayman Holdings Company, established in 2014. All intercompany accounts and transactions have been eliminated. The U.S. dollar is the functional currency for all of the Company's consolidated operations. All other accompanying financial statements for the years ended December 31, 2013 and 2012 include only the accounts of Versartis, Inc.
	Since inception, the Company has incurred net losses and negative cash flows from operations. At December 31, 2014, the Company had a deficit accumulated during the development stage of $111.3 million and working capital of $166.0 million. The Company expects to continue to incur losses from costs related to the continuation of research and development and administrative activities for the foreseeable future. Although management has been successful in raising capital in the past, most recently in January 2015, there can be no assurance that the Company will be successful or that any needed financing will be available in the future at terms acceptable to the Company.
	Segments
	The Company operates in one segment. Management uses one measurement of profitability and does not segregate its business for internal reporting. All long-lived assets are maintained in the United States of America.
	Reclassification
	Certain amounts within the consolidated balance sheet for the prior period have been reclassified to conform with the current period presentation. These reclassifications had no impact on the Company’s previously reported financial position.
	Reverse Stock Split
	On March 6, 2014, the Company effected a 1-for-11.5 reverse stock split of the Company’s issued and outstanding shares of common stock. The par value of the common stock was not adjusted as a result of the reverse stock split. All issued and outstanding common stock share and per share amounts included in the accompanying consolidated financial statements have been adjusted to reflect this reverse stock split for all periods presented, and the conversion ratio of the preferred stock was adjusted accordingly.
	Concentration of credit risk
	Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents. All of the Company’s cash and cash equivalents are held at four financial institutions that management believes are of high credit quality. Such deposits may, at times, exceed federally insured limits.
	Risk and Uncertainties
	The Company’s future results of operations involve a number of risks and uncertainties. Factors that could affect the Company’s future operating results and cause actual results to vary materially from expectations include, but are not limited to, uncertainty of results of clinical trials and reaching milestones, uncertainty of regulatory approval of the Company’s potential drug candidates, uncertainty of market acceptance of the Company’s products, competition from substitute products and larger companies, securing and protecting proprietary technology, strategic relationships and dependence on key individuals and sole source suppliers.
	Products developed by the Company require clearances from the U.S. Food and Drug Administration (“FDA”) or other international regulatory agencies prior to commercial sales. There can be no assurance that the products will receive the necessary clearances. If the Company was denied clearance, clearance was delayed or the Company was unable to maintain clearance, it could have a materially adverse impact on the Company.
	The Company expects to incur substantial operating losses for the next several years and will need to obtain additional financing in order to complete clinical studies and launch and commercialize any product candidates for which it receives regulatory approval. There can be no assurance that such financing will be available or will be at terms acceptable by the Company.
	Cash and cash equivalents
	The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents. At December 31, 2014 the Company’s cash and cash equivalents were held in four institutions in the United States and include deposits in money market funds which were unrestricted as to withdrawal or use. At December 31, 2013, the Company’s cash and cash equivalents were held in an institution in the United States and include deposits in a money market fund which was unrestricted as to withdrawal or use. Included in cash and cash equivalents at December 31, 2014 and December 31, 2013 was approximately $0.1 million of restricted cash held by a bank as security for the Company’s credit cards.
	Property and equipment, Net
	Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the assets, generally between three and five years. Leasehold improvements are amortized on a straight-line basis over the lesser of their useful life or the term of the lease. Maintenance and repairs are charged to expense as incurred, and improvements are capitalized. When assets are retired or otherwise disposed of, the cost and accumulated depreciation are removed from the consolidated balance sheet and any resulting gain or loss is reflected in operations in the period realized.
	Impairment of Long-Lived Assets
	The Company reviews property and equipment for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by the comparison of the carrying amount to the future net cash flows which the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the assets exceeds the fair value (i.e. determined through estimating projected discounted future net cash flows or other acceptable methods of determining fair value) arising from the asset. There have been no such impairments of long-lived assets as of December 31, 2014 and 2013 and the cumulative period from December 10, 2008 (date of inception) to December 31, 2014.
	Fair Value of Financial Instruments
	The carrying value of the Company’s cash and cash equivalents, prepaid expenses and other current assets, accounts payable and accrued liabilities approximate fair value due to the short-term nature of these items. Convertible preferred stock call option liability and convertible preferred stock warrant liability were carried at fair value.
	Fair value is defined as the exchange price that would be received for an asset or an exit price 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. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs.
	The fair value hierarchy defines a three-level valuation hierarchy for disclosure of fair value measurements as follows:
		Level I	Unadjusted quoted prices in active markets for identical assets or liabilities;
		Level II	Inputs other than quoted prices included within Level I that are observable, unadjusted quoted prices 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 III	Unobservable inputs that are supported by little or no market activity for the related assets or liabilities.
	The categorization of a financial instrument within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement.
	The Company’s financial instruments consist of Level I assets as of December 31, 2014 and 2013 and Level III liabilities as of December 31, 2013. Level I securities is comprised of highly liquid money market funds. Level III liabilities that are measured at fair value on a recurring basis consist of convertible preferred stock warrant liability and convertible preferred stock call option liability. The fair values of these instruments are measured using an option pricing model. Inputs used to determine estimated fair market value include the estimated fair value of the underlying stock at the valuation measurement date, the remaining expected term of the instrument, risk-free interest rates, expected dividends and the expected volatility.
	Preclinical and Clinical Trial Accruals
	The Company’s clinical trial accruals are based on estimates of patient enrollment and related costs at clinical investigator sites as well as estimates for the services received and efforts expended pursuant to contracts with multiple research institutions and clinical research organizations (“CROs”) that conduct and manage clinical trials on the Company’s behalf.
	The Company estimates preclinical and clinical trial expenses based on the services performed, pursuant to contracts with research institutions and clinical research organizations that conduct and manage preclinical studies and clinical trials on its behalf. In accruing service fees, the Company estimates the time period over which services will be performed and the level of patient enrollment and activity expended in each period. If the actual timing of the performance of services or the level of effort varies from the estimate, the Company will adjust the accrual accordingly. Payments made to third parties under these arrangements in advance of the receipt of the related services are recorded as prepaid expenses until the services are rendered.
	Convertible Preferred Stock Warrants
	The Company accounted for its convertible preferred stock warrants as liabilities based upon the characteristics and provisions of each instrument. Convertible preferred stock warrants classified as derivative liabilities were recorded on the Company’s consolidated balance sheet at their fair value on the date of issuance and revalued on each subsequent consolidated balance sheet, with fair value changes recognized as increases or reductions to other income (expense), net in the consolidated statements of operations.
	Prior to the IPO in March 2014, the Company had outstanding warrants which were classified as a liability and remeasured to fair value each reporting period. The Company had estimated the fair value of these liabilities using an option pricing model and assumptions that were based on the individual characteristics of the warrants on the valuation date, as well as assumptions for expected volatility, expected life, dividends, and risk-free interest rate. Immediately prior to the completion of the Company’s IPO in March 2014, all of the warrants were either exercised for cash or automatically net exercised for a total issuance of 158,179 shares of common stock, pursuant to the terms of the warrants. Just prior to the exercises, all outstanding warrants, covering 173,910 shares, were remeasured using the intrinsic value of the warrant computed as the difference between the $21.00 per share IPO price and the $5.17 per share exercise price of the warrant. The remeasurement of the fair value of these warrants from December 31, 2013 through the date of the conversion to a common stock warrant and following exercise resulted in a $2.3 million expense recorded to other income (expense), net in the consolidated statement of operations and comprehensive loss. The resulting fair value of approximately $2.8 million was reclassified to additional paid in capital upon completion of the IPO.
	Convertible Preferred Stock Call Option
	The Company determined that the Company’s obligation to issue, and the investors’ obligation to purchase, additional shares of the Company’s convertible preferred stock represented a freestanding financial instrument. The freestanding convertible preferred stock call option liability was initially recorded at fair value, with fair value changes recognized as increases or reductions to other income (expense), net in the consolidated statement of operations and comprehensive loss. At the time of the deemed exercise of the call option, the remaining value of the option was reclassified to additional paid in capital. Immediately prior to the Series D-2 financing completed in February 2014, the Company remeasured the fair value of the preferred stock call option liability associated with the Series D convertible preferred stock financing and recorded other expense of approximately $9.6 million in the consolidated statement of operations and comprehensive loss. Fair value was computed using a discount from the Company’s public offering price less the liquidation value of the underlying Series D convertible preferred stock.
	Convertible Preferred Stock
	The Company classified the convertible preferred stock as temporary equity on the balance sheets due to certain change in control events that are outside the Company’s control, including liquidation, sale or transfer of the Company, as holders of the convertible preferred stock can cause redemption of the shares. Upon the IPO in March 2014, all of the outstanding shares of convertible preferred stock automatically converted into 15,876,104 shares of common stock.
	In February 2014, the Company issued 48,758,857 shares of its Series E convertible preferred stock at a purchase price of $1.128 per share for an aggregate purchase price of approximately $55.0 million. The shares of convertible preferred stock automatically converted into 4,239,984 shares of common stock upon completion of the Company’s IPO. Pursuant to Accounting Standards Codification 470-20, Accounting for Convertible Securities with Beneficial Conversion Features or Contingently Adjustable Conversion Ratios, to Certain Convertible Instruments, the Company recorded a deemed dividend of approximately $25.6 million, which reflected a beneficial conversion feature on the underlying Series E preferred stock, in connection with the closing of the IPO on March 26, 2014.
	Research and development
	Research and development costs are charged to operations as incurred. Research and development costs include, but are not limited to, payroll and personnel expenses, laboratory supplies, consulting costs, external research and development expenses and allocated overhead, including rent, equipment depreciation, and utilities. Costs to acquire technologies to be used in research and development that have not reached technological feasibility and have no alternative future use are expensed to research and development costs when incurred.
	Income taxes
	The Company accounts for income taxes under the asset and liability approach. Under this method, deferred tax assets and liabilities are determined based on the difference between the consolidated financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to affect taxable income. Valuation allowances are established when necessary to reduce deferred tax assets to the amounts expected to be realized.
	The Company assesses all material positions taken in any income tax return, including all significant uncertain positions, in all tax years that are still subject to assessment or challenge by relevant taxing authorities. Assessing an uncertain tax position begins with the initial determination of the position’s sustainability and is measured at the largest amount of benefit that is greater than fifty percent likely of being realized upon ultimate settlement. As of each balance sheet date, unresolved uncertain tax positions must be reassessed, and the Company will determine whether (i) the factors underlying the sustainability assertion have changed and (ii) the amount of the recognized tax benefit is still appropriate. The recognition and measurement of tax benefits requires significant judgment. Judgments concerning the recognition and measurement of a tax benefit might change as new information becomes available.
	Stock-Based compensation
	For stock options granted to employees, the Company recognizes compensation expense for all stock-based awards based on the grant-date estimated fair value. The value of the portion of the award that is ultimately expected to vest is recognized as expense ratably over the requisite service period. The fair value of stock options is determined using the Black-Scholes option pricing model. The determination of fair value for stock-based awards on the date of grant using an option pricing model requires management to make certain assumptions regarding a number of complex and subjective variables.
	Stock-based compensation expense related to stock options granted to nonemployees is recognized based on the fair value of the stock options, determined using the Black-Scholes option pricing model, as they are earned. The awards generally vest over the time period the Company expects to receive services from the nonemployee.
	Consolidated Statement of Operations and Comprehensive Loss
	Comprehensive loss is defined as a change in equity of a business enterprise during a period, resulting from transactions from non-owner sources. There have been no items qualifying as other comprehensive loss and, therefore, for all periods presented, the Company’s comprehensive loss was the same as its reported net loss.
	Net Loss per Share of Common Stock
	Basic net loss per common share is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of common shares outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common shares and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, convertible preferred stock, convertible notes payable, stock options and convertible preferred stock warrants are considered to be potentially dilutive securities. Because the Company has reported a net loss for the years ended December 31, 2014, 2013 and 2012, diluted net loss per common share is the same as basic net loss per common share for those periods.
	Recent Accounting Pronouncements
	From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board, or FASB, or other standard setting bodies and adopted by us as of the specified effective date. Unless otherwise discussed, the impact of recently issued standards that are not yet effective will not have a material impact on the Company’s financial position or results of operations upon adoption.
	In June 2014, the Financial Accounting Standards Board issued Accounting Standards Update 2014-10, Elimination of Certain Financial Reporting Requirements, Including an Amendment to Variable Interest Entities Guidance in Topic 810, Consolidation, or ASU 2014-10, which eliminates the definition of a development stage entity, the development stage presentation and disclosure requirements under ASC 915, Development Stage Entities, and amends provisions of existing variable interest entity guidance under ASC 810, Consolidation. As a result of the changes, the financial statements of entities which meet the former definition of a development stage entity will no longer: (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. Furthermore, ASU 2014-10 clarifies disclosures about risks and uncertainties under ASC Topic 275, Risks and Uncertainties that apply to companies that have not commenced planned principal operations. Finally, variable interest entity rules no longer contain an exception for development stage entities and, as a result, development stage entities will have to be evaluated for consolidation in the same manner as non-development stage entities. These changes are effective for annual periods beginning after December 15, 2015 for public companies and are effective for annual periods beginning after December 15, 2016 for nonpublic entities. Early adoption is permitted for any annual reporting period or interim period for which the entity’s financial statements have not yet been issued (public business entities) or made available for issuance (other entities). The Company has elected not to early adopt and is currently evaluating the impact of adopting ASU 2014-10, but does not expect there to be any impact on its financial position, results of operations or cash flows.
	In August 2014, the FASB issued new guidance related to the disclosures around going concern. The new standard provides guidance around management’s responsibility to evaluate whether there is substantial doubt about an entity’s ability to continue as a going concern and to provide related footnote disclosures. The new standard is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2016. Early adoption is permitted. The Company will apply the guidance and disclosure provisions of the new standard upon adoption.

Balance_Sheet_Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2014
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Balance Sheet Components	3. Balance Sheet Components
	Prepaid expenses and other current assets (in thousands)
		December 31,
		2014			2013
	Preclinical and clinical	$	2,216		$	847
	Other		182			131
	Total	$	2,398		$	978
	Property and equipment, net (in thousands)
		December 31,
		2014			2013
	Equipment and furniture	$	706		$	50
	Buildings, leasehold and building improvements		132			21
	Construction-in-progress		—			—
			837			71
	Less: Accumulated depreciation and amortization		(124	)		(50	)
	Property and equipment, net	$	714		$	21
	Accrued liabilities (in thousands)
		December 31,
		2014			2013
	Payroll and related	$	1,737		$	539
	Preclinical and clinical		3,259			1,726
	Professional services		411			1,265
	Other		259			138
	Total	$	5,666		$	3,668

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements
	The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands):
		Fair Value Measurements at December 31, 2014
		Total			Level 1			Level 2		Level 3
	Assets
	Money market funds	$	160,125		$	160,125		$	—	$	—
		Fair Value Measurements at December 31, 2013
		Total			Level 1			Level 2		Level 3
	Assets
	Money market funds	$	12,761		$	12,761		$	—	$	—
	Liabilities
	Convertible preferred stock warrant liability	$	474		$	—		$	—	$	474
	Convertible preferred stock call option liability		21			—			—		21
	Total liabilities	$	495		$	—		$	—	$	495
	The fair value measurement of the convertible preferred stock warrant liability and convertible preferred stock call option liability is based on significant inputs not observed in the market and thus represents a Level 3 measurement. Level 3 instruments are valued based on unobservable inputs that are supported by little or no market activity and reflect the Company’s assumptions in measuring fair value. The Company’s estimated fair value of the convertible preferred stock warrant liability is calculated using an option pricing model and key assumptions including the probabilities of settlement scenarios, enterprise value, time to liquidity, risk-free interest rates, discount for lack of marketability and volatility. The Company’s estimated fair value of the preferred stock call option liability is calculated using an option pricing model and key assumptions including the estimated fair value of the Company’s preferred stock, risk-free interest rates and volatility and the probability of the closing of the future financing tranche. The estimates are based, in part, on subjective assumptions and could differ materially in the future.
	During the periods presented, the Company has not changed the manner in which it values liabilities that are measured at fair value using Level 3 inputs. The Company recognizes transfers between levels of the fair value hierarchy as of the end of the reporting period. There were no transfers within the hierarchy during the years ended December 31, 2014 or 2013.
	The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 financial instruments as follows:
		Convertible			Convertible
		preferred stock			preferred stock
		call option			warrant
		liability			liability
	Balance at January 1, 2014	$	21		$	474
	Fair value of call option liability recognized upon issuance of preferred stock		—			—
	Change in fair value recorded in other income (expense), net		9,560			2,278
	Conversion of preferred stock into common stock and reclassification to permanent equity		(9,581	)		(2,752	)
	Balance at December 31, 2014	$	—		$	—
		Convertible			Convertible
		preferred stock			preferred stock
		call option			warrant
		liability			liability
	Balance at January 1, 2013	$	—		$	433
	Issuance of financial instruments		1,116			—
	Fair value of call option liability recognized upon issuance of preferred stock		(126	)		—
	Change in fair value recorded in other income (expense), net		(969	)		41
	Balance at December 31, 2013	$	21		$	474

Convertible_Notes_Payable

Convertible Notes Payable	12 Months Ended
	Dec. 31, 2014
Text Block [Abstract]
Convertible Notes Payable	5. Convertible Notes Payable
	In 2010, the Company entered into convertible notes payable agreements (“2010 Notes”) with investors, in two tranches, for a total of $4.5 million. The terms of each of the 2010 Notes bear a fixed interest rate of 5%. In November 2010, the Company extended the maturity date of the 2010 Notes by two months to February 2011. The Company analyzed the amendment under the modification accounting guidance and concluded that the amendment did not result in a substantial modification. In February 2011, the outstanding 2010 Notes and accrued interest of $4.6 million were converted into 10,238,444 shares of Series B convertible preferred stock.
	In October 2012, the Company entered into convertible notes payable agreements (“2012 Notes”) with investors for a total of $4.5 million. The 2012 Notes bear a fixed interest rate of 8% accruing from the date of the issuance of 2012 Notes, with principal and unpaid interest payable on January 31, 2013. The 2012 Notes are convertible into shares of the next preferred stock financing at the purchase price of those shares, or into shares of Series B convertible preferred stock at $0.45 per share, at the option of the holder, or may be paid in cash in the event of a default.
	In January 2013, the holders of 2012 Notes converted principal of $4.5 million and $88,000 of accrued interest, into 10,195,552 shares of Series B preferred convertible stock at a price of $0.45 per share.

Convertible_Preferred_Stock_Wa

Convertible Preferred Stock Warrants	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Convertible Preferred Stock Warrants	6. Convertible Preferred Stock Warrants
	In connection with the convertible note purchase agreements (“2010 Notes”), the Company issued convertible preferred stock warrants equal to 20% of the shares issuable upon conversion of the 2010 Notes. Using an option pricing model with a volatility of 85%, term of 1.75 years and a risk-free interest rate of 0.53%, the fair value of the warrants was determined to be approximately $233,000 and was recorded as warrant liability and a debt discount against the 2010 Notes and amortized to interest expense over the term of the 2010 Notes. The convertible preferred stock warrants were exercised in 2012 for 2.0 million shares of Series B convertible preferred stock at an exercise price of $900,000.
	In connection with the convertible note purchase agreements (“2012 Notes”), the Company issued convertible preferred stock warrants equal to 20% of the shares issuable on conversion of the 2012 Notes. The convertible preferred stock warrants were exercisable into shares of the same class of convertible preferred stock issued upon conversion of the related 2012 Notes. The convertible preferred stock warrants had a five-year term and an expiration date of October 12, 2017. The estimated fair value of these warrants of $433,000 at issuance was recorded as a debt discount on the 2012 Notes, and amortized to interest expense using the effective interest method through the original maturity date in 2013. The convertible preferred stock warrants were valued using an option pricing model with a risk-free interest rate of 0.21%, volatility of 90%, and an expected life equal to 1.5 years. As of December 31, 2013, the fair value of the warrants was estimated to be $474,000.
	The 2012 warrants remained unexercised as of December 31, 2013. The terms of the warrants provided that they would expire at the earlier of (i) the closing of an initial public offering, (ii) a sale of the company or (iii) October 12, 2017; provided that if a holder of the warrants does not notify us of the holder’s intent to exercise or not to exercise the warrant prior to the expiration date, and the fair market value of the underlying shares on the expiration date is greater than the exercise price, then the holder will be deemed to have net exercised the warrant immediately prior to the expiration date. Upon the closing of the Company’s IPO, the warrants were exercised for a total of 158,179 shares of common stock.
	Prior to the IPO in March 2014, the Company had outstanding warrants which were classified as a liability and remeasured to fair value each reporting period. The Company had estimated the fair value of these liabilities using an option pricing model and assumptions that were based on the individual characteristics of the warrants on the valuation date, as well as assumptions for expected volatility, expected life, dividends, and risk-free interest rate. Immediately prior to the completion of the Company’s IPO in March 2014, all of the warrants were either exercised for cash or automatically net exercised for a total issuance of 158,179 shares of common stock, pursuant to the terms of the warrants. Just prior to the exercises, all outstanding warrants, covering 173,910 shares, were remeasured using the intrinsic value of the warrant computed as the difference between the $21.00 per share IPO price and the $5.17 per share exercise price of the warrant. The remeasurement of the fair value of these warrants from December 31, 2013 through the date of the conversion to a common stock warrant and following exercise resulted in a $2.3 million expense recorded to other income (expense), net in the consolidated statement of operations and comprehensive loss. The resulting fair value of approximately $2.8 million was reclassified to additional paid in capital upon completion of the IPO.
	The assumptions used to value the convertible preferred stock warrants were as follows:
		Year Ended
		December 31,
		2014	2013
	Expected term (in years)	-		1.1
	Expected volatility	-		75	%
	Risk-free interest rate	-		0.13	%
	Dividend yield	-		0	%

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies
	Facility Leases
	In August 2011, the Company signed an operating facility lease for its corporate office that includes approximately 5,740 square feet of office space in Redwood City, California. The lease term is for thirty months, commencing on October 16, 2011. The Company paid a security deposit of $55,000 for this facility lease in 2011, which was returned in 2014.
	In March 2014, the Company entered into an operating facility lease agreement to lease 12,943 square feet in Menlo Park, California for its new headquarters building for a period of thirty-nine months. The total obligation for the Company under this lease is approximately $2.1 million as of December 31, 2014.
	Rent expense was $520,000, $241,000 and $218,000 for the years ended December 31, 2014, 2013 and 2012, respectively, and $1,120,000 for the period from December 10, 2008 (inception) to December 31, 2014.
	As of December 31, 2014, the aggregate future minimum lease payments under the noncancellable operating lease arrangements are as follows (in thousands):
	Year Ended December 31, 2014
	2015	$	765
	2016		781
	2017		529
	2018		—
	2019		—
	Thereafter		—
		$	2,075
	Purchase Commitments
	The Company conducts research and development programs through a combination of internal and collaborative programs that include, among others, arrangements with contract manufacturing organizations and contract research organizations. The Company had contractual arrangements with these organizations including license agreements with milestone obligations and service agreements with obligations largely based on services performed.
	In the normal course of business, the Company enters into various firm purchase commitments related to certain preclinical and clinical studies. At December 31, 2014 the noncancellable portion of these commitments, in aggregate, totaled approximately $7.9 million.
	Contingencies
	In the normal course of business, the Company enters into contracts and agreements that contain a variety of representations and warranties and provide for general indemnifications. The Company’s exposure under these agreements is unknown because it involves claims that may be made against the Company in the future, but have not yet been made. The Company accrues a liability for such matters when it is probable that future expenditures will be made and such expenditures can be reasonably estimated.
	As of December 31, 2014 the Company is contingently committed to make development and sales-related milestone payments of up to $30.0 million under certain circumstances, and other payments of $10.0 million, as well as royalties relating to potential future product sales under the License Agreement with Amunix. The amount, timing and likelihood of these payments are unknown as they are dependent on the occurrence of future events that may or may not occur, including approval by the FDA of potential drug candidates.
	Indemnification
	In accordance with the Company’s amended and restated Certificate of Incorporation and amended and restated bylaws, the Company has indemnification obligations to its officers and directors for certain events or occurrences, subject to certain limits, while they are serving at the Company’s request in such capacity. There have been no claims to date and the Company has a director and officer insurance policy that may enable it to recover a portion of any amounts paid for future claims.
	Litigation
	The Company may from time to time be involved in legal proceedings arising from the normal course of business. There are no pending or threatened legal proceedings as of December 31, 2014.

Convertible_Preferred_Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Convertible Preferred Stock	8. Convertible Preferred Stock
	Convertible preferred stock (“preferred stock”) as of December 31, 2013 consisted of the following (in thousands, except share and per share data):
	At December 31, 2013 -
						Per Share
		Shares				Liquidation		Carrying
	Series	Authorized		Outstanding		Preference		Value
	Series A		22,000,000		22,000,000	$	0.45	$	9,900
	Series B		46,425,950		44,425,953		0.45		18,631
	Series C		36,444,444		36,444,444		0.56		19,301
	Series D-1		17,777,777		17,777,777		0.56		9,665
	Series D-2		13,168,291		—		0.76		—
			135,816,462		120,648,174			$	57,497
	Issuance of Series A preferred stock
	In December 2008, the Company entered into a stock purchase agreement with Amunix and other investors to issue 22,000,000 shares of Series A preferred stock at a purchase price of $1.00 per share in multiple closings.
	In consideration of Amunix entering into the License Agreement (Refer to Note 11) with the Company, the Company issued 11,000,000 shares of Series A convertible preferred stock in December 2008 to Amunix pursuant to the terms of the Series A Purchase Agreement and granted to Amunix the pro-rata right to participate in a future Series A closing if, at the sole discretion of certain investors, it is determined the Company has satisfied certain milestones, and the Board of Directors determines that the Company requires additional capital, for 2,500,000 shares at $1.00 per share. The pro rata right to participate in future Series A closing was determined to be issued to an entity under common control and as such the value of the option was initially recorded at a carrying value of $0. This right was waived by Amunix in 2010.
	The Company issued an aggregate of 8,000,000 shares of Series A preferred stock in May and December 2009 at a purchase price of $1.00 per share and further issued 3,000,000 shares in April 2010 as part of the third closing pursuant to the Series A convertible preferred stock purchase agreement.
	Issuance of Series B convertible preferred stock
	In February 2011, the Company entered into a Series B convertible preferred stock purchase agreement with certain investors. The Company issued 21,805,693 shares of Series B convertible preferred stock at a purchase price of $0.45 per share totaling $9.8 million in gross proceeds. Further, the 2010 convertible notes converted into 10,238,444 shares of Series B preferred upon the conversion of principal of $4.5 million and accrued interest of $107,300.
	In January and May 2012, the Company issued 12,861,110 shares of Series B convertible preferred stock at a purchase price of $0.45 per share in two subsequent closings and raised approximately $5.8 million in gross proceeds. The Company issued additional 2,000,000 shares of Series B convertible preferred stock upon the exercise of Series B convertible preferred stock warrants in exchange for $900,000.
	Issuance of Series C convertible preferred stock
	In January 2013, the Company entered into a Series C convertible preferred stock purchase agreement with certain investors. The Company issued 14,222,222 shares of Series C convertible preferred stock at a purchase price of $0.56 per share and raised approximately $8.0 million in gross proceeds.
	The Company issued additional 22,222,222 shares of Series C preferred stock in July 2013 at a price of $0.56 per share and raised $12.5 million in gross proceeds.
	Issuance of Series D convertible preferred stock
	In October 2013, the Company entered into a stock purchase agreement with investors to sell shares of Series D convertible preferred stock in two tranches. In accordance with the terms of the Series D purchase agreement, the Company authorized the sale and issuance of up to 30,946,068 shares of Series D stock. The Company issued 17,777,777 shares of Series D-1 at a purchase price of $0.56 per share and raised approximately $10.0 million in gross proceeds. In February 2014, the Company issued 13,168,291 shares of Series D-2 at a purchase price of $0.76 per share and received approximately $10.0 million in gross proceeds.
	Convertible preferred stock call option liability
	The preferred stock purchase agreements for Series A, B, C and D provided the investors the right to participate in future rounds of the respective series funding at a fixed price equal to the original issue price. These rights were provided concurrently with the issuance of the original preferred stock agreement (Series A convertible preferred stock had a separate option agreement). These liability classified convertible preferred stock call options have been determined to be freestanding financial instruments because they are freely transferable and separately exercisable.
	Series A convertible preferred stock call option liability
	On December 10, 2008, the Company sold a convertible preferred stock call option right to investors to participate in four future rounds of Series A convertible preferred financing. The Company recorded an initial call option liability in December 2008 and remeasured the convertible preferred stock liability immediately prior to exercise as well as at each balance sheet date. The convertible preferred stock call option right was exercised in three rounds of financing and was waived at the fourth round financing. The impact of the remeasurement of the option was negligible on a cumulative basis from December 10, 2008 to December 31, 2010.
	Series B convertible preferred stock call option liability
	The Company has recorded an initial convertible preferred stock call option liability in February 2011 upon the initial closing of the financing. Using an option pricing model with a volatility of 71%, expected term of 1 year and a risk-free interest rate of 0.28%, the fair value of the call option was determined to be approximately $1.4 million and was recorded as convertible preferred stock call option liability and netted against the issuance of Series B convertible preferred stock.
	The Company remeasured the convertible preferred stock call option liability at $349,000 at December 31, 2011 and recorded the change in fair value of $1.0 million in other income (expense), net. In 2012, the Company remeasured the convertible preferred stock call option liability immediately before exercise and recorded $88,000 in other income (expense), net and reclassified the fair value of $260,000 into preferred stock upon issuance of subsequent tranches of Series B convertible preferred stock.
	Series C convertible preferred stock call option liability
	The Company has recorded an initial convertible preferred stock call option liability in January 2013 upon the initial close of the financing. Using the option pricing model with a volatility of 56%, expected term of 0.5 years and a risk-free interest rate of 0.11%, the fair value of the convertible preferred stock call option was determined to be approximately $990,000 and was recorded as a call option liability and netted against the issuance of Series C convertible preferred stock.
	The investors exercised their right to participate in subsequent tranches in 2013 and accordingly the Company remeasured the convertible preferred stock call option liability just before exercise and recorded $864,000 in other income (expense), net and reclassified the fair value of $126,000 into preferred stock upon issuance of subsequent tranches of Series C preferred stock.
	Series D convertible preferred stock call option liability
	The Company has recorded an initial convertible preferred stock call option liability in October 2013 upon the initial close of the financing. Using the option pricing model with a volatility of 60%, expected term of 0.3 years and a risk-free interest rate of 0.2%, the fair value of the convertible preferred stock call option was determined to be approximately $126,000 and was recorded as a call option liability and netted against the issuance of Series D-1 convertible preferred stock. As of December 31, 2013, the fair value of the call option was determined to be $21,000, and the change in the fair value of $105,000 was recorded in other income (expense), net in the consolidated statement of operations and comprehensive loss.
	Modification to Series A rights and preferences
	In February 2011, the Company entered into an agreement with new investors to sell shares of Series B convertible preferred stock. In addition, the Company sold the rights to develop one of its two product candidates to Diartis in exchange for a $1.0 million note receivable. In connection with the Series B convertible preferred stock financing, the liquidation preference of the Series A convertible preferred stock was reduced from $1.00 per share to $0.45 per share, and the conversion ratio of Series A convertible preferred stock was reduced from $11.50 per share to $5.17 per share, the redemption feature was eliminated and a down-round feature was added to the rights and preferences of Series A convertible preferred stockholders. The Company considers an amendment which adds, deletes or significantly changes a material contractual term or fundamentally changes the nature of the preferred shares to be in the nature of an extinguishment. As there were significant changes in the liquidation preference and conversion rate, the removal of the Series A holder-controlled contingent redemption feature, and addition of down-round protection, the Company has considered these changes to be substantially different. At the time of the extinguishment, the Company removed the carrying value of the preferred stock of $22.0 million, and recorded the deemed fair value of the Series A convertible preferred stock after the change in the rights and preferences of $9.9 million, resulting in a net reduction of $12.1 million in the Series A preferred stock carrying value and an increase to deficit accumulated during the development stage.
	Accretion of preferred stock
	The Company recorded the convertible preferred stock at fair value on the dates of issuance. The Company classifies the convertible preferred stock outside of stockholders’ deficit because the shares contain liquidation features that are not solely within the Company’s control.
	The Series A shares were originally issued (before modification of rights and preferences upon issuance of Series B) with a contingent redemption feature, which allowed the holders to redeem their shares five years following the issuance date of the Series A preferred shares. As such, the Company has chosen to accrete Series A for change in redemption value with a change to accumulated deficit at the end of each reporting period. Accordingly, the Company has accreted $11.0 million during the cumulative periods ended December 31, 2010.
	During the years ended December 31, 2012 and 2013, the Company did not adjust the carrying values of the convertible preferred stock to the redemption values of such shares since a liquidation event was not probable. Subsequent adjustments to increase the carrying values to the ultimate redemption values will be made only when it becomes probable that such a liquidation event will occur.
	The rights, preferences and privileges of the convertible preferred stock as of December 31, 2013 are as follows:
	Dividends
	The holders of the Company’s convertible preferred stock are entitled to receive noncumulative dividends of $0.036 per share (as adjusted for stock splits, combinations, and reorganizations) per annum on each outstanding share of Series A and B preferred stock, and $0.045 per share for each outstanding share of Series C and D-1 preferred stock. Such dividends shall be payable only when and if declared by the Board of Directors. No dividends have been declared to date.
	Conversion
	Preferred stock is convertible, at the option of the holder, at any time, into fully paid, non-assessable shares of common stock at an initial conversion ratio of 11.5-to-one.
	The convertible preferred stock will automatically convert into common stock, at the then applicable conversion rate, in the event of either (i) the consent of a majority of certain holders of the then outstanding preferred stock, voting together as a class, or, if earlier, (ii) immediately before the closing of an underwritten initial public offering of the Company’s common stock pursuant to a registration statement on Form S-1 under the Securities Act of 1933, as amended, with aggregate proceeds of at least $50.0 million at a public offering price of at least $17.48 per share (adjusted for intervening common stock splits, stock dividends, combination, subdivision, recapitalizations or the like).
	Voting Rights
	The holders of convertible preferred stock will be entitled to that number of votes on all matters presented to stockholders equal to the number of shares of common stock then issuable upon conversion of such preferred stock.
	Liquidation
	In the event of any sale of substantially all of the assets, a merger, or liquidation, dissolution or winding up of the Company, the holders of Series A, B, C and D-1 convertible preferred stock will be entitled to receive in preference to the holders of common stock, $0.45, $0.45, $0.56 and $0.56 respectively, per share (as adjusted for stock splits, combinations, and reorganizations) plus declared and unpaid dividends, if any. Thereafter, the remaining assets of the Company will be distributed ratably to the holders of preferred stock and common stock on a pro rata basis.
	Deemed Liquidation
	A merger, acquisition, sale or lease of all or substantially all of the assets of the Company which will result in the Company’s stockholders immediately prior to such transaction not holding at least 50% of the voting power of the surviving, continuing or purchasing entity, shall be deemed to be a liquidation, dissolution or winding up. Upon this event, holders of convertible preferred stock shall receive their liquidation preference including any accrued and unpaid dividends as of the liquidation date.
	Protective Provisions
	Without first obtaining the approval of the Company’s board of directors, which approval must include the approval of a majority of the directors designated by certain holders of preferred stock, the Company will not (i) make specified investment, compensation or operating decisions; (ii) take certain actions involving the Company’s intellectual property and related agreements; (iii) consummate or consent to a liquidation event; (iv) increase or decrease the total number of authorized shares of any series or sub-series of preferred stock or of common stock or take certain other actions that may adversely affect the rights, preferences and privileges of the Company’s outstanding capital stock; (v) amend, waive, alter or repeal any provision of the Company’s certificate of incorporation or the company’s bylaws; (vi) purchase or redeem (or permit any subsidiary to purchase or redeem) or pay or declare a dividend or make any distribution on the capital stock of the Company, except in certain circumstances; (vii) purchase or acquire any other business or entity or create any subsidiary; (viii) incur specified types of indebtedness, (ix) adopt or amend any stock option scheme or any other equity incentive plan; (x) enter into certain transactions concerning the Company’s assets and intellectual property; (xi) enter into any transaction with any officer, director or other affiliate of the Company without required approvals; or (xii) change the authorized number of directors constituting the board of directors.
	Without first obtaining the written consent or affirmative vote of the holders of at least 66 2/3% of the then outstanding shares of Series D preferred stock, consenting or voting separately as a class, the Company will not: (i) amend, alter, or repeal any provision of the Company’s certificate of incorporation or bylaws in a manner that adversely affects the rights, preference or privileges of the Series D preferred stock; (ii) take certain other actions that may adversely affect the rights, preferences and privileges of the Series D preferred stock; (iii) declare or pay any dividend on any capital stock; (iv) consummate or consent to a liquidation event except in certain circumstances; or (v) increase or decrease the authorized number of shares of the Series D preferred stock.
	Without first obtaining the written consent or affirmative vote of certain stockholders specified in the certificate of incorporation, the Company will not: (i) consummate or consent to a liquidation event; (ii) amend, alter, or repeal any provision of the Company’s certificate of incorporation or bylaws; (iii) create, or authorize the creation of, or issue or obligate itself to issue shares of, any additional class or series or sub-series of capital stock (or any security convertible into or exercisable for any such capital stock) that ranks senior to or pari passu with any series or sub-series of preferred stock with respect to the distribution of assets on the liquidation, dissolution or winding up of the Company, the payment of dividends and rights of redemption; (iv) increase or decrease the authorized number of shares of common stock or preferred stock; (v) purchase or redeem (or permit any subsidiary to purchase or redeem) or pay or declare a dividend or make any distribution on the capital stock of the company, except in certain circumstances; (vi) incur or guarantee specified types of indebtedness; (vii) increase or decrease the size of the board of directors; (viii) increase the number of shares reserved for issuance under the Company’s equity incentive plans; (ix) alter or change the rights, preferences or privileges of any series or sub-series of preferred stock; (x) adopt or amend any stock option scheme or any other equity incentive plan; (xi) enter into certain transactions concerning the Company’s intellectual property; (xii) enter into any transaction with any officer, director or other affiliate of the Company without required approval; or (xiii) make any loan or advance, or grant any credit, to any employee or director of the Company or any subsidiary, except advances for travel expenses and similar expenditures to be incurred on behalf of the Company in the ordinary course of business.

Common_Stock

Common Stock	12 Months Ended
	Dec. 31, 2014
Text Block [Abstract]
Common Stock	9. Common Stock
	The Certificate of Incorporation, as amended, authorizes the Company to issue 50,000,000 shares of common stock. Common stockholders are entitled to dividends as and when declared by the Board of Directors, subject to the rights of holders of all classes of stock outstanding having priority rights as to dividends. There have been no dividends declared to date. The holder of each share of common stock is entitled to one vote.
	The Company had reserved common stock for future issuances as follows:
		December 31,
		2014		2013
	Conversion of convertible preferred stock		—		10,491,140
	Issuance of equity based awards under stock plan		991,401		9,533
	Issuance upon exercise of options under stock plan		2,723,366		1,403,655
	Issuance of restricted stock units under stock plan		185,514		—
	Issuance upon exercise of warrants to purchase Series B		—		173,912
	convertible preferred stock
	Total		3,900,281		12,078,240

Stock_Based_Awards

Stock Based Awards	12 Months Ended
	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Based Awards	10. Stock Based Awards
	2009 Equity Incentive Plan
	In February 2009, the Company adopted the Versartis, Inc. 2009 Stock Plan, which was amended in June 2011 (“2009 Plan”) for eligible employees, outside directors and consultants. The 2009 Plan provides for the granting of incentive stock options, non-statutory stock options, and stock purchase rights to acquire restricted stock. Terms of the stock option agreements, including vesting requirements, are determined by the board of directors, subject to the provisions in the 2009 Plan. Options granted by the Company generally vest over a period of four years and expire no later than ten years after the date of grant. Options may be exercised prior to vesting, subject to a right of repurchase by the Company. The board of directors determines the fair value of the underlying common stock at the time of the grant of each option. Upon the exercise of options, the Company issues new common stock from its authorized shares.
	Options under the 2009 Plan may be granted for periods of up to ten years. All options issued to date have had a ten year life. The exercise price of an ISO shall not be less than 100% of the estimated fair value of the shares on the date of grant, as determined by the Board of Directors. The exercise price of an ISO and NSO granted to a 10% shareholder shall not be less than 110% of the estimated fair value of the shares on the date of grant, respectively, as determined by the board of directors. The exercise price of a NSO shall not be less than the par value per share of common stock. To date, options granted generally vest over four years and vest at a rate of 25% upon the first anniversary of the issuance date and 1/36th per month thereafter.
	Upon adoption of the 2014 Equity Incentive Plan described below, no further grants will be made under the 2009 Plan.
	2014 Equity Incentive Plan
	In March 2014, the Company’s board of directors adopted, and the Company’s stockholders approved, the 2014 Equity Incentive Plan, or the 2014 Plan. The 2014 Plan became effective at the time of the initial public offering and is the successor to the 2009 Plan. The 2014 Plan provides for the grant of ISOs to employees and for the grant of NSOs, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance-based stock awards, performance-based cash awards and other forms of equity compensation to employees, directors and consultants. Additionally, the 2014 Plan provides for the grant of performance cash awards to employees, directors and consultants.
	Initially, the aggregate number of shares of common stock that may be issued pursuant to stock awards under the 2014 Plan after the initial public offering is approximately 4.1 million, which includes options outstanding under the 2009 Plan. The number of shares of common stock reserved for issuance under the 2014 Plan will automatically increase on January 1 of each year, beginning on January 1, 2015 (assuming the 2014 Plan becomes effective in 2014) and ending on and including January 1, 2024, by 4.5% of the total number of shares of the Company’s capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the board of directors. The maximum number of shares that may be issued upon the exercise of ISOs under the 2014 Plan is 12,000,000.
	The Company’s board of directors, or a duly authorized committee of the board of directors, will administer the 2014 Plan. The board of directors may also delegate to one or more of the Company’s officers the authority to (i) designate employees (other than officers) to receive specified stock awards, and (ii) determine the number of shares of our common stock to be subject to such stock awards. Subject to the terms of our 2014 Plan, the board of directors has the authority to determine the terms of awards, including recipients, the exercise, purchase or strike price of stock awards, if any, the number of shares subject to each stock award, the fair market value of a share of the Company’s common stock, the vesting schedule applicable to the awards, together with any vesting acceleration, and the form of consideration, if any, payable upon exercise or settlement of the award and the terms of the award agreements. Options granted under the 2014 Plan have a contractual life of ten years and generally vest over four years and vest at a rate of 25% upon the first anniversary of the issuance date and 1/36th per month thereafter.  The exercise price shall not be less than 100% of the fair market value of the shares on the date of grant.
	As of December 31, 2014, a total of 991,401 shares of common stock are available for future grant under the 2014 Plan.
	Activity under the Company’s stock option plans is set forth below:
										Weighted
										Average
								Weighted		Remaining		Aggregate
		Shares						Average		Contractual		Intrinsic
		Available			Number of			Exercise		Life		Value
		for Grant			Shares			Price		(in years)		(in thousands)
	Balance at January 1, 2013		168,571			462,471		$	1.31
	Additional shares authorized		803,590			—			—
	Options granted		(974,459	)		974,459			2.16
	Options exercised		—			(21,444	)		1.27
	Options cancelled		11,831			(11,831	)		1.3
	Balances, December 31, 2013		9,533			1,403,655		$	1.9
	Additional shares authorized		2,531,915			—			—
	Options granted		(1,471,142	)		1,471,142			18.68
	Restricted stock units granted		(185,514	)		—			—
	Options exercised		—			(44,822	)		1.32
	Options cancelled		106,609			(106,609	)		18.43
	Balances, December 31, 2014		991,401			2,723,366		$	10.33		8.8	$	36,006
	Vested and expected to vest as of December 31, 2014		—			2,651,182		$	10.27		8.8	$	35,141
	Exercisable as of December 31, 2014		—			648,058		$	3.04		7.7	$	12,858
	The intrinsic values of outstanding, vested and exercisable options were determined by multiplying the number of shares by the difference in exercise price of the options and the fair value of the common stock. The intrinsic value of stock options exercised during the years ended December 31, 2014, 2013, and 2012 was $0.8 million, zero, and zero, respectively.
	The following table summarizes information with respect to stock options outstanding and currently exercisable and vested as of December 31, 2014:
								Options Exercisable
		Options Outstanding						and Vested
					Weighted Average					Weighted Average
					Remaining					Remaining
	Range of	Number			Contractual			Number		Contractual
	Exercise Prices	Outstanding			Life (in Years)			Outstanding		Life (in Years)
	$1.27 - $1.61		769,755			7.5			471,420		7.5
	2.53		570,927			9			146,484		9
	3.34 - 18.66		576,987			9.2			1,087		9.2
	18.73 - 31.96		795,697			9.6			29,067		9.6
	34		10,000			9.5			—		9.5
			2,723,366						648,058
	Stock Options Granted to Employees
	During the years ended December 31, 2014, 2013, and 2012 the Company granted stock options to employees to purchase shares of common stock with a weighted-average grant date fair value of $16.90, $1.60, and $1.04 per share, respectively. The fair value is being expensed over the vesting period of the options, which is usually 4 years on a straight line basis as the services are being provided. No tax benefits were realized from options and other share-based payment arrangements during the periods.
	As of December 31, 2014, total unrecognized employee stock-based compensation was $20.2 million, which is expected to be recognized over the weighted-average remaining vesting period of 3.1 years.
	The fair value of employee stock options was estimated using the following weighted average assumptions:
		Year Ended December 31,
		2014			2013			2012
	Expected volatility		84.90%			89.10%			91.30%
	Risk-free interest rate		1.90%			1.70%			1.10%
	Dividend yield		0.00%			0.00%			0.00%
	Expected life (in years)		6.1			6.1			6.1
	Determining Fair Value of Stock Options
	The fair value of each grant of stock options was determined by the Company using the methods and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment to determine.
	Expected Volatility – The expected stock price volatility assumption was determined by examining the historical volatilities of a group of industry peers, as the Company did not have any trading history for the Company’s common stock. The Company will continue to analyze the historical stock price volatility and expected term assumptions as more historical data for the Company’s common stock becomes available.
	Expected Term – The expected term of stock options represents the weighted average period the stock options are expected to be outstanding. For option grants that are considered to be “plain vanilla”, the Company has opted to use the simplified method for estimating the expected term as provided by the Securities and Exchange Commission. The simplified method calculates the expected term as the average time-to-vesting and the contractual life of the options. For other option grants, the expected term is derived from the Company’s historical data on employee exercises and post-vesting employment termination behavior taking into account the contractual life of the award.
	Risk-Free Interest Rate – The risk free rate assumption was based on the U.S. Treasury instruments with terms that were consistent with the expected term of the Company’s stock options.
	Expected Dividend – The expected dividend assumption was based on the Company’s history and expectation of dividend payouts.
	Forfeiture Rate – Forfeitures were estimated based on historical experience.
	Fair Value of Common Stock – The fair value of the shares of common stock underlying the stock options has historically been the responsibility of and determined by the Company’s board of directors. Because there had been no public market for the Company’s common stock prior to the initial public offering, the board of directors determined the fair value of common stock at the time of grant of the option by considering a number of objective and subjective factors including independent third-party valuations of the Company’s common stock, sales of convertible preferred stock to unrelated third parties, operating and financial performance, the lack of liquidity of capital stock and general and industry specific economic outlook, amongst other factors. Since the initial public offering in March 2014, the fair value of the underlying common stock is based upon quoted prices on the NASDAQ Global Select Market.
	Stock-based compensation expense, net of estimated forfeitures, is reflected in the statements of operations and comprehensive loss as follows (in thousands):
									Cumulative
									Period From
									December 10,
									2008  (Date of
									Inception) to
	Year Ended December 31,								December 31,
	2014			2013			2012		2014
Operating Expenses
Research and development	$	1,230		$	124		$	91	$	1,599
General and administrative		3,411			91			50		3,590
Total	$	4,641		$	215		$	141	$	5,189
2014 Employee Stock Purchase Plan
The board of directors adopted, and the Company’s stockholders approved, the 2014 Employee Stock Purchase Plan, or the ESPP, in March 2014. The ESPP became effective on March 20, 2014.
The maximum aggregate number of shares of common stock that may be issued under the ESPP is 150,000 shares (subject to adjustment to reflect any split of our common stock). Additionally, the number of shares of common stock reserved for issuance under the ESPP will increase automatically each year, beginning on January 1, 2015 and continuing through and including January 1, 2024, by the lesser of (i) 1% of the total number of shares of our common stock outstanding on December 31 of the preceding calendar year; and (ii) 300,000 shares of common stock (subject to adjustment to reflect any split of our common stock). The board of directors may act prior to the first day of any calendar year to provide that there will be no January 1 increase or that the increase will be for a lesser number of shares than would otherwise occur. Shares subject to purchase rights granted under the ESPP that terminate without having been exercised in full will not reduce the number of shares available for issuance under the ESPP.
An employee may not be granted rights to purchase stock under the ESPP if such employee (i) immediately after the grant would own stock possessing 5% or more of the total combined voting power or value of the Company’s common stock, or (ii) holds rights to purchase stock under the ESPP that would accrue at a rate that exceeds $25,000 worth of our stock for each calendar year that the rights remain outstanding.
The administrator may approve offerings with a duration of not more than 27 months, and may specify one or more shorter purchase periods within each offering. Each offering will have one or more purchase dates on which shares of common stock will be purchased for the employees who are participating in the offering. The administrator, in its discretion, will determine the terms of offerings under the ESPP.
The ESPP permits participants to purchase shares of our common stock through payroll deductions with up to 15% of their earnings. The purchase price of the shares will be not less than 85% of the lower of the fair market value of our common stock on the first day of an offering or on the date of purchase.
We estimated the fair value of our employees’ stock purchase rights under the ESPP using the Black-Scholes model with the following weighted-average assumptions:
	Year Ended December 31,
	2014			2013
Expected volatility		54.90%		-
Risk-free interest rate		0.06%		-
Dividend yield		0.00%		-
Expected life (in years)		0.5		-
Restricted Stock Units
Restricted stock units are shares of common stock which are forfeited if the employee leaves the Company prior to vesting. These stock units offer employees the opportunity to earn shares of the Company’s stock over time, rather than options that give the employee the right to purchase stock at a set price. As a result of these restricted stock units, the Company recognized $0.5 million in compensation expense during the year ended December 31, 2014.  As all of the restricted stock vests through 2015 and beyond, the Company will continue to recognize stock based compensation expense related to the grants of these restricted stock units. If all of the remaining restricted stock units that were granted in 2014 vest, the Company will recognize approximately $4.3 million in compensation expense over a weighted average remaining period of 3.5 years. However, no compensation expense will be recognized for restricted stock units that do not vest.
A summary of the Company’s restricted stock activity is presented in the following tables:
				Weighted
				Average
	Number of			Grant Date
	Shares			Fair Value
Restricted Stock Units
Unvested at December 31, 2013		—		$	—
Granted		185,514			26.77
Vested		—			—
Forfeited/canceled		—			—
Unvested at December 31, 2014		185,514		$	26.77

Related_Party_Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Related Party Transactions	11. Related Party Transactions
	Since inception the Company has entered into multiple agreements with Amunix which (i) with its affiliates, owns 10% of the Company’s preferred stock outstanding at December 31, 2013, and (ii) is represented on the Company’s Board of Directors. These agreements between the Company and Amunix include the following:
	·	License Agreement effective December 29, 2008, as amended, (“License Agreement”), pursuant to which the Company has the right to develop three products, with the option to develop up to three additional products in exchange for certain additional financial considerations. Amunix granted the Company a worldwide, exclusive, revocable sub-licensable right and licensed its intellectual property for the Company to research, test and develop these products. The License Agreement obligates the Company to pay to Amunix certain future royalties related to these products. One of these products, and the option to develop one additional product, were sold to Diartis on December 30, 2010. The agreement was further amended at the close of the Company’s Series C preferred stock financing on January 7, 2013, to clarify the technology included in the License Agreement;
		The Company will pay Amunix additional consideration, in either cash or the Company’s stock, for additional targets selected by the Company. The Company will also pay up to $30.0 million of milestone payments to Amunix, under certain circumstances;
	·	Joint Research Agreement effective November 13, 2009, as amended and combined with the License Agreement, establishing the process by which new targets will be identified and subsequently developed by the parties. In particular, the respective ownership of new inventions by the parties under various scenarios is contemplated. Overall, during the term of this agreement, the Company agreed to assign to Amunix its rights to all joint patents, and all the Company’s patents that are directed to compositions, processes and methods of use or recombinant PEGylation (“rPEG”) technology and/or targets comprising rPEG;
	·	Service Agreement (“Service Agreement”) effective December 29, 2008, as amended, setting forth the terms under which Amunix has agreed to make covered products and marketed products for the Company as contemplated by the Licensing Agreement. Under the Service Agreement, Amunix agreed to undertake and complete the research, development and other services related to the covered products and marketed products as are reasonably requested by the Company from time to time. The specific milestones, deliverables, specifications and other terms with respect to any particular services project are to be detailed in mutually agreeable statements of work, which the parties are to negotiate (reasonably and in good faith) and execute promptly after the Company’s request for services;
	·	Office sublease effective September 15, 2009 which expired in November 2011;
	·	Employee Cost Sharing Agreement, effective January 19, 2010, for the term of one year, whereby the Company makes certain employees available to Amunix to perform business development and finance services with respect to a Consulting Agreement. Amunix, Inc. will reimburse the Company a percentage of the employees’ salary and benefits. This Agreement expired on February 1, 2012.
	The aggregate operating expenses included in the consolidated statements of operations and comprehensive loss pertaining to these agreements were approximately $0, $0, and $74,000 for the years ended December 31, 2014, 2013 and 2012, respectively, and $7.4 million for the period from December 10, 2008 (date of inception) to December 31, 2014. There were no amounts receivable or payable at December 31, 2014 pertaining to these agreements.
	Effective May 12, 2009, the Company entered into a consulting agreement with Mark de Boer, Ph.D. Mr. de Boer (i) is a partner with Index Ventures which owns 24% of the Company’s preferred stock outstanding at December 31, 2013, and (ii) had served as Chairman of the Company’s Board of Directors. The consulting fees were incurred in the ordinary course of business, and were zero for the year ended December 31, 2012 and $120,000 for the period from December 10, 2008 (date of inception) to December 31, 2014. The consulting agreement terminated on December 10, 2010 and Mr. de Boer left the Board of Directors on February 14, 2011. In May 2011, the Company declined to repurchase unvested option shares from Mr. de Boer, thereby accelerating the vesting of 10,185 shares of common stock. The Company recognized an additional $1,000 of stock compensation expense in 2011 related to this transaction.
	The Company entered into a Services Agreement with Diartis dated February 14, 2011 for a term equal to the term of the Amunix License Agreement (see above). Under this agreement, the Company provided certain administrative services to Diartis related to the management of its Phase 1a Human Clinical Trial for the treatment of certain metabolic diseases, as a clinical research organization.
	In March 2013, the Company ended its relationship with Diartis and terminated the Services Agreement between the companies.
	The aggregate operating expenses included in the consolidated statements of operations and comprehensive loss pertaining to these agreements were approximately $0, $0, and $1.4 million for the years ended December 31, 2014, 2013 and 2012, respectively, and $3.9 million for the period from December 10, 2008 (inception) to December 31, 2014. There were no receivables due from Diartis or accounts payable to third parties related to the conduct of business for Diartis at December 31, 2014 or December 31, 2013.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	12. Income Taxes
	There is no provision for federal income taxes in 2014, 2013 and 2012 and for the period from December 10, 2008 (date of inception) to December 31, 2013, respectively.
	Income (loss) before income taxes is attributed to the following geographic locations for the year ended December 31, 2014, 2013 and 2012 (in thousand):
		December 31,
		2014			2013				2012
	United States	$	(49,850	)	$	(18,497	)		$	(13,217	)
	Foreign		(7,663	)		—				—
	Income (loss) before income taxes	$	(57,513	)	$	(18,497	)		$	(13,217	)
	Income tax expense in 2014, 2013 and 2012 differed from the amount expected by applying the statutory federal tax rate to the income or loss before taxes as summarized below:
		December 31,
		2014			2013				2012
	Federal tax benefit at statutory rate		34	%		34	%			34	%
	State tax benefit net of federal effect		—			—				—
	Change in valuation allowance		(23	)%		(42	)%			(33	)%
	Research and development credits		2	%		9	%			—
	Non-deductible warrant		(7	)%		—				—
	Foreign loss not benefitted		(5	)%		—				—
	Non-deductible expenses and other		(1	)%		(1	)%			(1	)%
	Total		0	%		0	%			0	%
	Deferred income taxes reflect the net tax effects of net operating loss and tax credit carryforwards and temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of the Company’s net deferred tax assets at December 31, 2014 and 2013 are as follows (in thousands):
		December 31,
		2014			2013
	Net operating loss carry forwards	$	32,143					$	21,414
	Research and development tax credits		2,776						2,414
	Accruals and reserves		1,693						47
	Depreciation and amortization		33						89
	Total deferred tax assets		36,645						23,964
	Less: Valuation allowance		(36,645			)			(23,964		)
	Net deferred tax assets	$	—					$	—
	The Company’s accounting for deferred taxes involves the evaluation of a number of factors concerning the realizability of its net deferred tax assets. The Company primarily considered such factors as its history of operating losses, the nature of the Company’s deferred tax assets, and the timing, likelihood and amount, if any, of future taxable income during the periods in which those temporary differences and carryforwards become deductible. At present, the Company does not believe that it is more likely than not that the deferred tax assets will be realized; accordingly, a full valuation allowance has been established and no deferred tax asset is shown in the accompanying balance sheets.
	The valuation allowance increased by approximately $12.7 million and $9.1 million in 2014 and 2013, respectively.
	At December 31, 2014, the Company has net operating loss carryforwards for federal income tax purposes of approximately $85.7 million and federal research and development tax credits of approximately $840,000, which begin to expire in 2029. The Company also has net operating loss carryforwards for state income tax purposes of approximately $55.1 million, which begin to expire in 2029, and state research and development tax credits of approximately $698,000 which have no expiration date. Additionally, the Company has an Orphan Drug Credit of approximately $1.8 million for federal income tax purposes, which begins to expire in 2033.
	Utilization of net operating losses and tax credit carryforwards may be limited by the “ownership change” rules, as defined in Section 382 of the Internal Revenue Code (any such limitation, a “Section 382 limitation”). Similar rules may apply under state tax laws. The Company has performed an analysis to determine whether an “ownership change” occurred from inception to the Company’s initial public offering in March 2014. Based on this analysis, management determined that the Company did experience historical ownership changes of greater than 50% during this period. Therefore, the utilization of a portion of the Company’s net operating losses and credit carryforwards is currently limited. However, these Section 382 limitations are not expected to result in a permanent loss of the net operating losses and credit carryforwards. As such, a reduction to the Company’s gross deferred tax asset for its net operating loss and tax credit carryforwards is not necessary prior to considering the valuation allowance. In the event the Company experiences any subsequent changes in ownership, the amount of net operating losses and research and development credit carryforwards useable in any taxable year could be limited and may expire unutilized.
	The Company follows the provisions of FASB Accounting Standards Codification 740-10 (ASC 740-10), Accounting for Uncertainty in Income Taxes. ASC 740-10 prescribes a comprehensive model for the recognition, measurement, presentation and disclosure in consolidated financial statements of uncertain tax positions that have been taken or expected to be taken on a tax return. No liability related to uncertain tax positions is recorded in the consolidated financial statements. At December 31, 2014, 2013 and 2012, the Company’s reserve for unrecognized tax benefits is approximately $332,000, $287,000 and $73,000, respectively. Due to the full valuation allowance at December 31, 2014, current adjustments to the unrecognized tax benefit will have no impact on the Company’s effective income tax rate; any adjustments made after the valuation allowance is released will have an impact on the tax rate. The Company does not anticipate any significant change in its uncertain tax positions within 12 months of this reporting date. The Company includes penalties and interest expense related to income taxes as a component of other expense and interest expense, respectively, as necessary.
	Because the statute of limitations does not expire until after the net operating loss and credit carryforwards are actually used, the statute is open for all tax years from inception, that is, for the period from December 10, 2008 (date of inception) to December 31, 2014 and forward for federal and state tax purposes.
	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands):
		Amount
	Balance at January 1, 2012	$	62
	Increase/(Decrease) of unrecognized tax benefits related to current year		11
	Balance at December 31, 2012	$	73
	Increase/(Decrease) of unrecognized tax benefits taken in prior years		31
	Increase/(Decrease) of unrecognized tax benefits related to current year		183
	Balance at December 31, 2013	$	287
	Decreases based on tax positions taken during a prior period		(102	)
	Increases based on tax positions taken during a current period		147
	Balance at December 31, 2014	$	332
	All tax years remain open for examination by federal and state tax authorities.

Defined_Contribution_Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2014
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	13. Defined Contribution Plan
	The Company sponsors a 401(k) Plan, which stipulates that eligible employees can elect to contribute to the 401(k) Plan, subject to certain limitations of eligible compensation. The Company may match employee contributions in amounts to be determined at the Company’s sole discretion. To date, the Company has not made any matching contributions.

Net_Loss_per_Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Net Loss per Share	14. Net loss per share
	The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands, except per share data):
			December 31,
			2014			2013			2012
	Net loss attributable to common stockholders - basic		$	(83,072	)	$	(18,497	)	$	(13,217	)
	and diluted
	Weighted-average shares outstanding			18,921,533			450,061			130,718
	Less: weighted average shares subject to repurchase			—			(61	)		(15,499	)
	Weighted-average shares used to compute basic and			18,921,533			450,000			115,219
	diluted net loss per share
	Basic and diluted net loss per common share		$	(4.39	)	$	(41.10	)	$	(114.71	)
	Basic net loss attributable to common stockholders per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common shares outstanding for the period. Diluted net loss attributable to common stockholders per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common shares and dilutive common stock equivalents outstanding for the period, determined using the treasury-stock method and the as-if converted method, for convertible securities, if inclusion of these is dilutive. Because the Company has reported a net loss for the years ended December 31, 2014, 2013 and 2012, diluted net loss per common share is the same as basic net loss per common share for those years.
	The following potentially dilutive securities outstanding at the end of the years presented have been excluded from the computation of diluted shares outstanding:
			December 31,
			2014			2013				2012
	Convertible preferred stock			—			10,491,140				5,991,757
	Warrants to purchase convertible preferred stock (1)			—			173,912				173,912
	Options to purchase common stock			2,723,366			1,403,655				462,471
	Convertible notes			—			—				885,024
	Restricted stock units			185,514			—				—
	-1	Assumes exercise of warrants to purchase convertible preferred stock at $5.17 per share.

Subsequent_Event

Subsequent Event	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Event	15. Subsequent Event
	In January 2015, the Company completed a secondary public offering of shares of its common stock, pursuant to which the Company issued 4,999,999 shares of common stock, which includes shares issued pursuant to the underwriters’ exercise of their over-allotment option, and received net proceeds of approximately $80.3 million, after underwriting discounts, commissions and estimated offering expenses.

Quarterly_Results_Unaudited

Quarterly Results (Unaudited)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Quarterly Results (Unaudited)	16. Quarterly Results (Unaudited)
	The following table is in thousands, except per share amounts:
		Quarters Ended
		March 31,			June 30,			September 30,			December 31,
		2014			2014			2014			2014
	Consolidated Statement of operations data:
	Operating expenses
	Research and development	$	4,869		$	5,622		$	10,515		$	11,602
	General and administrative		2,714			2,877			3,577			4,338
	Total operating expenses		7,583			8,499			14,092			15,940
	Loss from operations		(7,583	)		(8,499	)		(14,092	)		(15,940	)
	Interest income		—			40			50			43
	Interest expense		—			—			—			—
	Other income (expense), net		(11,843	)		(141	)		208			244
	Net loss and comprehensive loss		(19,426	)		(8,600	)		(13,834	)		(15,653	)
	Deemed dividend related to beneficial conversion		(25,559	)		-			-			-
	feature of convertible preferred stock
	Net loss attributable to common stockholders	$	(44,985	)	$	(8,600	)	$	(13,834	)	$	(15,653	)
	Net loss per basic and diluted share attributable to	$	(16.13	)	$	(0.36	)	$	(0.57	)	$	(0.65	)
	common stockholders
	Weighted-average common shares used to compute		2,788,087			24,194,808			24,194,808			24,215,018
	basic and diluted net loss per share
		Year Ended December 31,
		March 31,			June 30,			September 30,			December 31,
		2013			2013			2013			2013
	Statement of operations data:
	Operating expenses
	Research and development	$	3,499		$	3,326		$	4,576		$	3,454
	General and administrative		682			656			685			2,405
	Total operating expenses		4,181			3,982			5,261			5,859
	Loss from operations		(4,181	)		(3,982	)		(5,261	)		(5,859	)
	Interest income		—			—			—			1
	Interest expense		(128	)		—			—			—
	Other income (expense), net		290			327			252			44
	Net loss and comprehensive loss		(4,019	)		(3,655	)		(5,009	)		(5,814	)
	Deemed dividend related to beneficial conversion		-			-			-			-
	feature of convertible preferred stock
	Net loss attributable to common stockholders	$	(4,019	)	$	(3,655	)	$	(5,009	)	$	(5,814	)
	Net loss per basic and diluted share attributable to	$	(30.23	)	$	(24.74	)	$	(19.98	)	$	(4.62	)
	common stockholders
	Weighted-average common shares used to compute		132,969			147,713			250,745			1,257,250
	basic and diluted net loss per share

Schedule_II_Valuation_and_Qual

Schedule II Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2014
Valuation And Qualifying Accounts [Abstract]
Schedule II Valuation and Qualifying Accounts
	VERSARTIS, INC.
	Schedule II: Valuation and Qualifying Accounts
	(in thousands)
		Balance at
		beginning of		Additions/charged				Balance at
		period		to expense		Deductions		end of period
	Year ended December 31, 2014
	Valuation allowances for deferred tax assets	$	23,964	$	12,681	$	—	$	36,645
	Year ended December 31, 2013
	Valuation allowances for deferred tax assets	$	14,852	$	9,112	$	—	$	23,964
	Year ended December 31, 2012
	Valuation allowances for deferred tax assets	$	9,681	$	5,171	$	—	$	14,852

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Basis of Presentation and Use of Estimates	Basis of Presentation and Use of Estimates
	The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”). The preparation of the accompanying consolidated financial statements in accordance with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of expenses during the reporting period. Actual results could differ from those estimates.
	The accompanying financial statements are consolidated for the year ended December 31, 2014 and include the accounts of Versartis, Inc. and its wholly-owned subsidiary, Versartis Cayman Holdings Company, established in 2014. All intercompany accounts and transactions have been eliminated. The U.S. dollar is the functional currency for all of the Company's consolidated operations. All other accompanying financial statements for the years ended December 31, 2013 and 2012 include only the accounts of Versartis, Inc.
	Since inception, the Company has incurred net losses and negative cash flows from operations. At December 31, 2014, the Company had a deficit accumulated during the development stage of $111.3 million and working capital of $166.0 million. The Company expects to continue to incur losses from costs related to the continuation of research and development and administrative activities for the foreseeable future. Although management has been successful in raising capital in the past, most recently in January 2015, there can be no assurance that the Company will be successful or that any needed financing will be available in the future at terms acceptable to the Company.
Segment	Segments
	The Company operates in one segment. Management uses one measurement of profitability and does not segregate its business for internal reporting. All long-lived assets are maintained in the United States of America.
Reclassification	Reclassification
	Certain amounts within the consolidated balance sheet for the prior period have been reclassified to conform with the current period presentation. These reclassifications had no impact on the Company’s previously reported financial position.
Reverse Stock Split	Reverse Stock Split
	On March 6, 2014, the Company effected a 1-for-11.5 reverse stock split of the Company’s issued and outstanding shares of common stock. The par value of the common stock was not adjusted as a result of the reverse stock split. All issued and outstanding common stock share and per share amounts included in the accompanying consolidated financial statements have been adjusted to reflect this reverse stock split for all periods presented, and the conversion ratio of the preferred stock was adjusted accordingly.
Concentration of credit risk	Concentration of credit risk
	Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents. All of the Company’s cash and cash equivalents are held at four financial institutions that management believes are of high credit quality. Such deposits may, at times, exceed federally insured limits.
Risk and Uncertainties	Risk and Uncertainties
	The Company’s future results of operations involve a number of risks and uncertainties. Factors that could affect the Company’s future operating results and cause actual results to vary materially from expectations include, but are not limited to, uncertainty of results of clinical trials and reaching milestones, uncertainty of regulatory approval of the Company’s potential drug candidates, uncertainty of market acceptance of the Company’s products, competition from substitute products and larger companies, securing and protecting proprietary technology, strategic relationships and dependence on key individuals and sole source suppliers.
	Products developed by the Company require clearances from the U.S. Food and Drug Administration (“FDA”) or other international regulatory agencies prior to commercial sales. There can be no assurance that the products will receive the necessary clearances. If the Company was denied clearance, clearance was delayed or the Company was unable to maintain clearance, it could have a materially adverse impact on the Company.
	The Company expects to incur substantial operating losses for the next several years and will need to obtain additional financing in order to complete clinical studies and launch and commercialize any product candidates for which it receives regulatory approval. There can be no assurance that such financing will be available or will be at terms acceptable by the Company.
Cash and cash equivalents	Cash and cash equivalents
	The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents. At December 31, 2014 the Company’s cash and cash equivalents were held in four institutions in the United States and include deposits in money market funds which were unrestricted as to withdrawal or use. At December 31, 2013, the Company’s cash and cash equivalents were held in an institution in the United States and include deposits in a money market fund which was unrestricted as to withdrawal or use. Included in cash and cash equivalents at December 31, 2014 and December 31, 2013 was approximately $0.1 million of restricted cash held by a bank as security for the Company’s credit cards.
Property and equipment, Net	Property and equipment, Net
	Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the assets, generally between three and five years. Leasehold improvements are amortized on a straight-line basis over the lesser of their useful life or the term of the lease. Maintenance and repairs are charged to expense as incurred, and improvements are capitalized. When assets are retired or otherwise disposed of, the cost and accumulated depreciation are removed from the consolidated balance sheet and any resulting gain or loss is reflected in operations in the period realized.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets
	The Company reviews property and equipment for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by the comparison of the carrying amount to the future net cash flows which the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the carrying amount of the assets exceeds the fair value (i.e. determined through estimating projected discounted future net cash flows or other acceptable methods of determining fair value) arising from the asset. There have been no such impairments of long-lived assets as of December 31, 2014 and 2013 and the cumulative period from December 10, 2008 (date of inception) to December 31, 2014.
Fair Value of Financial Instruments	Fair Value of Financial Instruments
	The carrying value of the Company’s cash and cash equivalents, prepaid expenses and other current assets, accounts payable and accrued liabilities approximate fair value due to the short-term nature of these items. Convertible preferred stock call option liability and convertible preferred stock warrant liability were carried at fair value.
	Fair value is defined as the exchange price that would be received for an asset or an exit price 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. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs.
	The fair value hierarchy defines a three-level valuation hierarchy for disclosure of fair value measurements as follows:
		Level I	Unadjusted quoted prices in active markets for identical assets or liabilities;
		Level II	Inputs other than quoted prices included within Level I that are observable, unadjusted quoted prices 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 III	Unobservable inputs that are supported by little or no market activity for the related assets or liabilities.
	The categorization of a financial instrument within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement.
	The Company’s financial instruments consist of Level I assets as of December 31, 2014 and 2013 and Level III liabilities as of December 31, 2013. Level I securities is comprised of highly liquid money market funds. Level III liabilities that are measured at fair value on a recurring basis consist of convertible preferred stock warrant liability and convertible preferred stock call option liability. The fair values of these instruments are measured using an option pricing model. Inputs used to determine estimated fair market value include the estimated fair value of the underlying stock at the valuation measurement date, the remaining expected term of the instrument, risk-free interest rates, expected dividends and the expected volatility.
Preclinical and Clinical Trial Accruals	Preclinical and Clinical Trial Accruals
	The Company’s clinical trial accruals are based on estimates of patient enrollment and related costs at clinical investigator sites as well as estimates for the services received and efforts expended pursuant to contracts with multiple research institutions and clinical research organizations (“CROs”) that conduct and manage clinical trials on the Company’s behalf.
	The Company estimates preclinical and clinical trial expenses based on the services performed, pursuant to contracts with research institutions and clinical research organizations that conduct and manage preclinical studies and clinical trials on its behalf. In accruing service fees, the Company estimates the time period over which services will be performed and the level of patient enrollment and activity expended in each period. If the actual timing of the performance of services or the level of effort varies from the estimate, the Company will adjust the accrual accordingly. Payments made to third parties under these arrangements in advance of the receipt of the related services are recorded as prepaid expenses until the services are rendered.
Convertible Preferred Stock Warrants	Convertible Preferred Stock Warrants
	The Company accounted for its convertible preferred stock warrants as liabilities based upon the characteristics and provisions of each instrument. Convertible preferred stock warrants classified as derivative liabilities were recorded on the Company’s consolidated balance sheet at their fair value on the date of issuance and revalued on each subsequent consolidated balance sheet, with fair value changes recognized as increases or reductions to other income (expense), net in the consolidated statements of operations.
	Prior to the IPO in March 2014, the Company had outstanding warrants which were classified as a liability and remeasured to fair value each reporting period. The Company had estimated the fair value of these liabilities using an option pricing model and assumptions that were based on the individual characteristics of the warrants on the valuation date, as well as assumptions for expected volatility, expected life, dividends, and risk-free interest rate. Immediately prior to the completion of the Company’s IPO in March 2014, all of the warrants were either exercised for cash or automatically net exercised for a total issuance of 158,179 shares of common stock, pursuant to the terms of the warrants. Just prior to the exercises, all outstanding warrants, covering 173,910 shares, were remeasured using the intrinsic value of the warrant computed as the difference between the $21.00 per share IPO price and the $5.17 per share exercise price of the warrant. The remeasurement of the fair value of these warrants from December 31, 2013 through the date of the conversion to a common stock warrant and following exercise resulted in a $2.3 million expense recorded to other income (expense), net in the consolidated statement of operations and comprehensive loss. The resulting fair value of approximately $2.8 million was reclassified to additional paid in capital upon completion of the IPO.
Convertible Preferred Stock Call Option	Convertible Preferred Stock Call Option
	The Company determined that the Company’s obligation to issue, and the investors’ obligation to purchase, additional shares of the Company’s convertible preferred stock represented a freestanding financial instrument. The freestanding convertible preferred stock call option liability was initially recorded at fair value, with fair value changes recognized as increases or reductions to other income (expense), net in the consolidated statement of operations and comprehensive loss. At the time of the deemed exercise of the call option, the remaining value of the option was reclassified to additional paid in capital. Immediately prior to the Series D-2 financing completed in February 2014, the Company remeasured the fair value of the preferred stock call option liability associated with the Series D convertible preferred stock financing and recorded other expense of approximately $9.6 million in the consolidated statement of operations and comprehensive loss. Fair value was computed using a discount from the Company’s public offering price less the liquidation value of the underlying Series D convertible preferred stock.
Convertible Preferred Stock	Convertible Preferred Stock
	The Company classified the convertible preferred stock as temporary equity on the balance sheets due to certain change in control events that are outside the Company’s control, including liquidation, sale or transfer of the Company, as holders of the convertible preferred stock can cause redemption of the shares. Upon the IPO in March 2014, all of the outstanding shares of convertible preferred stock automatically converted into 15,876,104 shares of common stock.
	In February 2014, the Company issued 48,758,857 shares of its Series E convertible preferred stock at a purchase price of $1.128 per share for an aggregate purchase price of approximately $55.0 million. The shares of convertible preferred stock automatically converted into 4,239,984 shares of common stock upon completion of the Company’s IPO. Pursuant to Accounting Standards Codification 470-20, Accounting for Convertible Securities with Beneficial Conversion Features or Contingently Adjustable Conversion Ratios, to Certain Convertible Instruments, the Company recorded a deemed dividend of approximately $25.6 million, which reflected a beneficial conversion feature on the underlying Series E preferred stock, in connection with the closing of the IPO on March 26, 2014.
Research and development	Research and development
	Research and development costs are charged to operations as incurred. Research and development costs include, but are not limited to, payroll and personnel expenses, laboratory supplies, consulting costs, external research and development expenses and allocated overhead, including rent, equipment depreciation, and utilities. Costs to acquire technologies to be used in research and development that have not reached technological feasibility and have no alternative future use are expensed to research and development costs when incurred.
Income taxes	Income taxes
	The Company accounts for income taxes under the asset and liability approach. Under this method, deferred tax assets and liabilities are determined based on the difference between the consolidated financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to affect taxable income. Valuation allowances are established when necessary to reduce deferred tax assets to the amounts expected to be realized.
	The Company assesses all material positions taken in any income tax return, including all significant uncertain positions, in all tax years that are still subject to assessment or challenge by relevant taxing authorities. Assessing an uncertain tax position begins with the initial determination of the position’s sustainability and is measured at the largest amount of benefit that is greater than fifty percent likely of being realized upon ultimate settlement. As of each balance sheet date, unresolved uncertain tax positions must be reassessed, and the Company will determine whether (i) the factors underlying the sustainability assertion have changed and (ii) the amount of the recognized tax benefit is still appropriate. The recognition and measurement of tax benefits requires significant judgment. Judgments concerning the recognition and measurement of a tax benefit might change as new information becomes available.
Stock-Based compensation	Stock-Based compensation
	For stock options granted to employees, the Company recognizes compensation expense for all stock-based awards based on the grant-date estimated fair value. The value of the portion of the award that is ultimately expected to vest is recognized as expense ratably over the requisite service period. The fair value of stock options is determined using the Black-Scholes option pricing model. The determination of fair value for stock-based awards on the date of grant using an option pricing model requires management to make certain assumptions regarding a number of complex and subjective variables.
	Stock-based compensation expense related to stock options granted to nonemployees is recognized based on the fair value of the stock options, determined using the Black-Scholes option pricing model, as they are earned. The awards generally vest over the time period the Company expects to receive services from the nonemployee.
Consolidated Statement of Operations and Comprehensive Loss	Consolidated Statement of Operations and Comprehensive Loss
	Comprehensive loss is defined as a change in equity of a business enterprise during a period, resulting from transactions from non-owner sources. There have been no items qualifying as other comprehensive loss and, therefore, for all periods presented, the Company’s comprehensive loss was the same as its reported net loss.
Net Loss per Share of Common Stock	Net Loss per Share of Common Stock
	Basic net loss per common share is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of common shares outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common shares and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, convertible preferred stock, convertible notes payable, stock options and convertible preferred stock warrants are considered to be potentially dilutive securities. Because the Company has reported a net loss for the years ended December 31, 2014, 2013 and 2012, diluted net loss per common share is the same as basic net loss per common share for those periods.
Recent Accounting Pronouncements	Recent Accounting Pronouncements
	From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board, or FASB, or other standard setting bodies and adopted by us as of the specified effective date. Unless otherwise discussed, the impact of recently issued standards that are not yet effective will not have a material impact on the Company’s financial position or results of operations upon adoption.
	In June 2014, the Financial Accounting Standards Board issued Accounting Standards Update 2014-10, Elimination of Certain Financial Reporting Requirements, Including an Amendment to Variable Interest Entities Guidance in Topic 810, Consolidation, or ASU 2014-10, which eliminates the definition of a development stage entity, the development stage presentation and disclosure requirements under ASC 915, Development Stage Entities, and amends provisions of existing variable interest entity guidance under ASC 810, Consolidation. As a result of the changes, the financial statements of entities which meet the former definition of a development stage entity will no longer: (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. Furthermore, ASU 2014-10 clarifies disclosures about risks and uncertainties under ASC Topic 275, Risks and Uncertainties that apply to companies that have not commenced planned principal operations. Finally, variable interest entity rules no longer contain an exception for development stage entities and, as a result, development stage entities will have to be evaluated for consolidation in the same manner as non-development stage entities. These changes are effective for annual periods beginning after December 15, 2015 for public companies and are effective for annual periods beginning after December 15, 2016 for nonpublic entities. Early adoption is permitted for any annual reporting period or interim period for which the entity’s financial statements have not yet been issued (public business entities) or made available for issuance (other entities). The Company has elected not to early adopt and is currently evaluating the impact of adopting ASU 2014-10, but does not expect there to be any impact on its financial position, results of operations or cash flows.
	In August 2014, the FASB issued new guidance related to the disclosures around going concern. The new standard provides guidance around management’s responsibility to evaluate whether there is substantial doubt about an entity’s ability to continue as a going concern and to provide related footnote disclosures. The new standard is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2016. Early adoption is permitted. The Company will apply the guidance and disclosure provisions of the new standard upon adoption.

Balance_Sheet_Components_Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2014
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets (in thousands)
		December 31,
		2014			2013
	Preclinical and clinical	$	2,216		$	847
	Other		182			131
	Total	$	2,398		$	978
Property and equipment, net	Property and equipment, net (in thousands)
		December 31,
		2014			2013
	Equipment and furniture	$	706		$	50
	Buildings, leasehold and building improvements		132			21
	Construction-in-progress		—			—
			837			71
	Less: Accumulated depreciation and amortization		(124	)		(50	)
	Property and equipment, net	$	714		$	21
Accrued Liabilities	Accrued liabilities (in thousands)
		December 31,
		2014			2013
	Payroll and related	$	1,737		$	539
	Preclinical and clinical		3,259			1,726
	Professional services		411			1,265
	Other		259			138
	Total	$	5,666		$	3,668

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Financial Instruments Measured at Fair Value on Recurring Basis	The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands):
		Fair Value Measurements at December 31, 2014
		Total			Level 1			Level 2		Level 3
	Assets
	Money market funds	$	160,125		$	160,125		$	—	$	—
		Fair Value Measurements at December 31, 2013
		Total			Level 1			Level 2		Level 3
	Assets
	Money market funds	$	12,761		$	12,761		$	—	$	—
	Liabilities
	Convertible preferred stock warrant liability	$	474		$	—		$	—	$	474
	Convertible preferred stock call option liability		21			—			—		21
	Total liabilities	$	495		$	—		$	—	$	495
Summary of Changes in Fair Value of Financial Instruments	The following table sets forth a summary of the changes in the fair value of the Company’s Level 3 financial instruments as follows:
		Convertible			Convertible
		preferred stock			preferred stock
		call option			warrant
		liability			liability
	Balance at January 1, 2014	$	21		$	474
	Fair value of call option liability recognized upon issuance of preferred stock		—			—
	Change in fair value recorded in other income (expense), net		9,560			2,278
	Conversion of preferred stock into common stock and reclassification to permanent equity		(9,581	)		(2,752	)
	Balance at December 31, 2014	$	—		$	—
		Convertible			Convertible
		preferred stock			preferred stock
		call option			warrant
		liability			liability
	Balance at January 1, 2013	$	—		$	433
	Issuance of financial instruments		1,116			—
	Fair value of call option liability recognized upon issuance of preferred stock		(126	)		—
	Change in fair value recorded in other income (expense), net		(969	)		41
	Balance at December 31, 2013	$	21		$	474

Convertible_Preferred_Stock_Wa1

Convertible Preferred Stock Warrants (Tables)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Assumptions Used to Value Convertible Preferred Stock Warrants	The assumptions used to value the convertible preferred stock warrants were as follows:
		Year Ended
		December 31,
		2014	2013
	Expected term (in years)	-		1.1
	Expected volatility	-		75	%
	Risk-free interest rate	-		0.13	%
	Dividend yield	-		0	%

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Lease Payments Required Under Non-Cancellable Operating Leases	As of December 31, 2014, the aggregate future minimum lease payments under the noncancellable operating lease arrangements are as follows (in thousands):
	Year Ended December 31, 2014
	2015	$	765
	2016		781
	2017		529
	2018		—
	2019		—
	Thereafter		—
		$	2,075

Convertible_Preferred_Stock_Ta

Convertible Preferred Stock (Tables)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Schedule of Convertible Preferred Stock	Convertible preferred stock (“preferred stock”) as of December 31, 2013 consisted of the following (in thousands, except share and per share data):
	At December 31, 2013 -
						Per Share
		Shares				Liquidation		Carrying
	Series	Authorized		Outstanding		Preference		Value
	Series A		22,000,000		22,000,000	$	0.45	$	9,900
	Series B		46,425,950		44,425,953		0.45		18,631
	Series C		36,444,444		36,444,444		0.56		19,301
	Series D-1		17,777,777		17,777,777		0.56		9,665
	Series D-2		13,168,291		—		0.76		—
			135,816,462		120,648,174			$	57,497

Common_Stock_Tables

Common Stock (Tables)	12 Months Ended
	Dec. 31, 2014
Text Block [Abstract]
Summary of Reserved Common Stock for Future Issuances	The Company had reserved common stock for future issuances as follows:
		December 31,
		2014		2013
	Conversion of convertible preferred stock		—		10,491,140
	Issuance of equity based awards under stock plan		991,401		9,533
	Issuance upon exercise of options under stock plan		2,723,366		1,403,655
	Issuance of restricted stock units under stock plan		185,514		—
	Issuance upon exercise of warrants to purchase Series B		—		173,912
	convertible preferred stock
	Total		3,900,281		12,078,240

Stock_Based_Awards_Tables

Stock Based Awards (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	Activity under the Company’s stock option plans is set forth below:
										Weighted
										Average
								Weighted		Remaining		Aggregate
		Shares						Average		Contractual		Intrinsic
		Available			Number of			Exercise		Life		Value
		for Grant			Shares			Price		(in years)		(in thousands)
	Balance at January 1, 2013		168,571			462,471		$	1.31
	Additional shares authorized		803,590			—			—
	Options granted		(974,459	)		974,459			2.16
	Options exercised		—			(21,444	)		1.27
	Options cancelled		11,831			(11,831	)		1.3
	Balances, December 31, 2013		9,533			1,403,655		$	1.9
	Additional shares authorized		2,531,915			—			—
	Options granted		(1,471,142	)		1,471,142			18.68
	Restricted stock units granted		(185,514	)		—			—
	Options exercised		—			(44,822	)		1.32
	Options cancelled		106,609			(106,609	)		18.43
	Balances, December 31, 2014		991,401			2,723,366		$	10.33		8.8	$	36,006
	Vested and expected to vest as of December 31, 2014		—			2,651,182		$	10.27		8.8	$	35,141
	Exercisable as of December 31, 2014		—			648,058		$	3.04		7.7	$	12,858
Stock Options Outstanding and Exercisable under Stock Option Plans	The following table summarizes information with respect to stock options outstanding and currently exercisable and vested as of December 31, 2014:
								Options Exercisable
		Options Outstanding						and Vested
					Weighted Average					Weighted Average
					Remaining					Remaining
	Range of	Number			Contractual			Number		Contractual
	Exercise Prices	Outstanding			Life (in Years)			Outstanding		Life (in Years)
	$1.27 - $1.61		769,755			7.5			471,420		7.5
	2.53		570,927			9			146,484		9
	3.34 - 18.66		576,987			9.2			1,087		9.2
	18.73 - 31.96		795,697			9.6			29,067		9.6
	34		10,000			9.5			—		9.5
			2,723,366						648,058
Summary of Fair Value of Employee Stock Options	The fair value of employee stock options was estimated using the following weighted average assumptions:
		Year Ended December 31,
		2014			2013			2012
	Expected volatility		84.90%			89.10%			91.30%
	Risk-free interest rate		1.90%			1.70%			1.10%
	Dividend yield		0.00%			0.00%			0.00%
	Expected life (in years)		6.1			6.1			6.1
Schedule of Estimated Stock-Based Compensation Expense	Stock-based compensation expense, net of estimated forfeitures, is reflected in the statements of operations and comprehensive loss as follows (in thousands):
										Cumulative
										Period From
										December 10,
										2008  (Date of
										Inception) to
		Year Ended December 31,								December 31,
		2014			2013			2012		2014
	Operating Expenses
	Research and development	$	1,230		$	124		$	91	$	1,599
	General and administrative		3,411			91			50		3,590
	Total	$	4,641		$	215		$	141	$	5,189
Summary of Fair Value of Employees Stock Purchase Rights Under ESPP Using Black-Scholes Model with Following Weighted-Average Assumptions	We estimated the fair value of our employees’ stock purchase rights under the ESPP using the Black-Scholes model with the following weighted-average assumptions:
		Year Ended December 31,
		2014			2013
	Expected volatility		54.90%		-
	Risk-free interest rate		0.06%		-
	Dividend yield		0.00%		-
	Expected life (in years)		0.5		-
Summary of Restricted Stock Activity	A summary of the Company’s restricted stock activity is presented in the following tables:
					Weighted
					Average
		Number of			Grant Date
		Shares			Fair Value
	Restricted Stock Units
	Unvested at December 31, 2013		—		$	—
	Granted		185,514			26.77
	Vested		—			—
	Forfeited/canceled		—			—
	Unvested at December 31, 2014		185,514		$	26.77

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Schedule of Income (Loss) Before Income Taxes Attributed to Geographic Locations	Income (loss) before income taxes is attributed to the following geographic locations for the year ended December 31, 2014, 2013 and 2012 (in thousand):
		December 31,
		2014			2013				2012
	United States	$	(49,850	)	$	(18,497	)		$	(13,217	)
	Foreign		(7,663	)		—				—
	Income (loss) before income taxes	$	(57,513	)	$	(18,497	)		$	(13,217	)
Summary of Statutory Federal Tax Rate to Income or Loss before Taxes	Income tax expense in 2014, 2013 and 2012 differed from the amount expected by applying the statutory federal tax rate to the income or loss before taxes as summarized below:
		December 31,
		2014			2013				2012
	Federal tax benefit at statutory rate		34	%		34	%			34	%
	State tax benefit net of federal effect		—			—				—
	Change in valuation allowance		(23	)%		(42	)%			(33	)%
	Research and development credits		2	%		9	%			—
	Non-deductible warrant		(7	)%		—				—
	Foreign loss not benefitted		(5	)%		—				—
	Non-deductible expenses and other		(1	)%		(1	)%			(1	)%
	Total		0	%		0	%			0	%
Significant Components of Net Deferred Tax Assets	Significant components of the Company’s net deferred tax assets at December 31, 2014 and 2013 are as follows (in thousands):
		December 31,
		2014			2013
	Net operating loss carry forwards	$	32,143					$	21,414
	Research and development tax credits		2,776						2,414
	Accruals and reserves		1,693						47
	Depreciation and amortization		33						89
	Total deferred tax assets		36,645						23,964
	Less: Valuation allowance		(36,645			)			(23,964		)
	Net deferred tax assets	$	—					$	—
Reconciliation of Amount of Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands):
		Amount
	Balance at January 1, 2012	$	62
	Increase/(Decrease) of unrecognized tax benefits related to current year		11
	Balance at December 31, 2012	$	73
	Increase/(Decrease) of unrecognized tax benefits taken in prior years		31
	Increase/(Decrease) of unrecognized tax benefits related to current year		183
	Balance at December 31, 2013	$	287
	Decreases based on tax positions taken during a prior period		(102	)
	Increases based on tax positions taken during a current period		147
	Balance at December 31, 2014	$	332

Net_Loss_per_Share_Tables

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Summary of Computation of Basic and Diluted Net Loss Per Share	The following table summarizes the computation of basic and diluted net loss per share attributable to common stockholders of the Company (in thousands, except per share data):
			December 31,
			2014			2013			2012
	Net loss attributable to common stockholders - basic		$	(83,072	)	$	(18,497	)	$	(13,217	)
	and diluted
	Weighted-average shares outstanding			18,921,533			450,061			130,718
	Less: weighted average shares subject to repurchase			—			(61	)		(15,499	)
	Weighted-average shares used to compute basic and			18,921,533			450,000			115,219
	diluted net loss per share
	Basic and diluted net loss per common share		$	(4.39	)	$	(41.10	)	$	(114.71	)
Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Shares Outstanding	The following potentially dilutive securities outstanding at the end of the years presented have been excluded from the computation of diluted shares outstanding:
			December 31,
			2014			2013				2012
	Convertible preferred stock			—			10,491,140				5,991,757
	Warrants to purchase convertible preferred stock (1)			—			173,912				173,912
	Options to purchase common stock			2,723,366			1,403,655				462,471
	Convertible notes			—			—				885,024
	Restricted stock units			185,514			—				—
	-1	Assumes exercise of warrants to purchase convertible preferred stock at $5.17 per share.

Quarterly_Results_Unaudited_Ta

Quarterly Results (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	The following table is in thousands, except per share amounts:
		Quarters Ended
		March 31,			June 30,			September 30,			December 31,
		2014			2014			2014			2014
	Consolidated Statement of operations data:
	Operating expenses
	Research and development	$	4,869		$	5,622		$	10,515		$	11,602
	General and administrative		2,714			2,877			3,577			4,338
	Total operating expenses		7,583			8,499			14,092			15,940
	Loss from operations		(7,583	)		(8,499	)		(14,092	)		(15,940	)
	Interest income		—			40			50			43
	Interest expense		—			—			—			—
	Other income (expense), net		(11,843	)		(141	)		208			244
	Net loss and comprehensive loss		(19,426	)		(8,600	)		(13,834	)		(15,653	)
	Deemed dividend related to beneficial conversion		(25,559	)		-			-			-
	feature of convertible preferred stock
	Net loss attributable to common stockholders	$	(44,985	)	$	(8,600	)	$	(13,834	)	$	(15,653	)
	Net loss per basic and diluted share attributable to	$	(16.13	)	$	(0.36	)	$	(0.57	)	$	(0.65	)
	common stockholders
	Weighted-average common shares used to compute		2,788,087			24,194,808			24,194,808			24,215,018
	basic and diluted net loss per share
		Year Ended December 31,
		March 31,			June 30,			September 30,			December 31,
		2013			2013			2013			2013
	Statement of operations data:
	Operating expenses
	Research and development	$	3,499		$	3,326		$	4,576		$	3,454
	General and administrative		682			656			685			2,405
	Total operating expenses		4,181			3,982			5,261			5,859
	Loss from operations		(4,181	)		(3,982	)		(5,261	)		(5,859	)
	Interest income		—			—			—			1
	Interest expense		(128	)		—			—			—
	Other income (expense), net		290			327			252			44
	Net loss and comprehensive loss		(4,019	)		(3,655	)		(5,009	)		(5,814	)
	Deemed dividend related to beneficial conversion		-			-			-			-
	feature of convertible preferred stock
	Net loss attributable to common stockholders	$	(4,019	)	$	(3,655	)	$	(5,009	)	$	(5,814	)
	Net loss per basic and diluted share attributable to	$	(30.23	)	$	(24.74	)	$	(19.98	)	$	(4.62	)
	common stockholders
	Weighted-average common shares used to compute		132,969			147,713			250,745			1,257,250
	basic and diluted net loss per share

Formation_and_Business_of_the_1

Formation and Business of the Company - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended	73 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Initial Public Offering [Line Items]
Business incorporated date		10-Dec-08
Proceeds from issuance of common stock in initial public offering, net of issuance costs	$132,100	$132,137	$132,137
Common stock, shares authorized	50,000,000	50,000,000	50,000,000	15,652,174
Convertible preferred stock, shares authorized	5,000,000	5,000,000	5,000,000	135,816,462
IPO [Member]
Initial Public Offering [Line Items]
Initial public offering shares of common stock issued	6,900,000

Summary_of_Significant_Account2

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended	12 Months Ended			73 Months Ended	1 Months Ended
	Mar. 31, 2014	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Feb. 28, 2014
			Segment
Significant Accounting Policies [Line Items]
Accumulated deficit			$111,259,000	$53,746,000		$111,259,000
Working capital			166,000,000			166,000,000
Number of operating segment			1
Reverse stock split			1-for-11.5
Cash and cash equivalents maturity period			Three months or less
Cash and cash equivalents of restricted cash held for credit cards			100,000	100,000		100,000
Impairments of long-lived assets			0	0		0
Inception date			10-Dec-08
Number of securities issued upon exercise of warrants	158,179	158,179
Remeasurement of outstanding warrants	173,910
Warrants to purchase convertible preferred stock, exercise price	$5.17	$5.17
Convertible preferred stock shares automatically converted into common stock	15,876,104	15,876,104
Purchase price of convertible preferred stock			64,793,000	29,956,000	5,765,000	120,113,000
Deemed dividend related to beneficial conversion feature of convertible preferred stock		25,559,000	25,559,000			25,559,000
Series D convertible preferred stock [Member]
Significant Accounting Policies [Line Items]
Remeasurement of preferred stock							9,600,000
Series E convertible preferred stock [Member]
Significant Accounting Policies [Line Items]
Convertible preferred stock shares automatically converted into common stock							48,758,857
Convertible preferred stock, Purchase price							$1.13
Purchase price of convertible preferred stock							55,000,000
Number of common stock automatically converted from convertible preferred stock upon completion of the Company's IPO							4,239,984
Deemed dividend related to beneficial conversion feature of convertible preferred stock							25,600,000
IPO [Member]
Significant Accounting Policies [Line Items]
Warrants to purchase convertible preferred stock, exercise price	$21	$21
Remeasurement [Member]
Significant Accounting Policies [Line Items]
Expense related to change in fair value	2,300,000
Reclassified to additional paid in capital	$2,800,000
Minimum [Member]
Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives			3 years
Maximum [Member]
Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives			5 years
Percentage for the amount of benefit realized upon ultimate settlement			50.00%	50.00%

Balance_Sheet_Components_Prepa

Balance Sheet Components - Prepaid Expenses and Other Current Assets (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Prepaid Expense And Other Assets [Abstract]
Preclinical and clinical	$2,216	$847
Other	182	131
Total	$2,398	$978

Balance_Sheet_Components_Prope

Balance Sheet Components - Property and Equipment, net (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Property Plant And Equipment [Line Items]
Property and equipment, gross	$837	$71
Less: Accumulated depreciation and amortization	-124	-50
Property and equipment, net	714	21
Equipment and Furniture [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	706	50
Buildings, Leasehold and Building Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$132	$21

Balance_Sheet_Components_Accru

Balance Sheet Components - Accrued Liabilities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Payables And Accruals [Abstract]
Payroll and related	$1,737	$539
Preclinical and clinical	3,259	1,726
Professional services	411	1,265
Other	259	138
Total	$5,666	$3,668

Fair_Value_Measurements_Financ

Fair Value Measurements - Financial Instruments Measured at Fair Value on Recurring Basis (Detail) (Recurring [Member], USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities, fair value		$495
Conversion of preferred stock warrant liability [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities, fair value		474
Conversion of preferred stock call option liability [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities, fair value		21
Money market funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	160,125	12,761
Level 1 [Member] | Money market funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets, fair value	160,125	12,761
Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities, fair value		495
Level 3 [Member] | Conversion of preferred stock warrant liability [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities, fair value		474
Level 3 [Member] | Conversion of preferred stock call option liability [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities, fair value		$21

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fair Value Disclosures [Abstract]
Transfers within the hierarchy	$0	$0

Fair_Value_Measurements_Summar

Fair Value Measurements - Summary of Changes in Fair Value of Financial Instruments (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Conversion of preferred stock call option liability [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$21
Issuance of financial instruments		1,116
Fair value of call option liability recognized upon issuance of preferred stock		-126
Change in fair value recorded in other income (expense), net	9,560	-969
Conversion of preferred stock into common stock and reclassification to permanent equity	-9,581
Ending balance		21
Conversion of preferred stock warrant liability [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	474	433
Change in fair value recorded in other income (expense), net	2,278	41
Conversion of preferred stock into common stock and reclassification to permanent equity	-2,752
Ending balance		$474

Convertible_Notes_Payable_Addi

Convertible Notes Payable - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended		1 Months Ended
	Feb. 28, 2011	Dec. 31, 2014	Dec. 31, 2010	Jan. 31, 2013	Oct. 31, 2012
			Tranche
Debt Instrument [Line Items]
Maturity date		28-Feb-11
Amount of debt converted into shares	$4,600,000
Series B convertible preferred stock [Member]
Debt Instrument [Line Items]
Number of shares issued in exchange for Notes payable	10,238,444
2010 Notes [Member]
Debt Instrument [Line Items]
Convertible notes payable, current			4,500,000
Number of tranches			2
Fixed interest rate			5.00%
2012 Notes [Member]
Debt Instrument [Line Items]
Convertible notes payable, current					4,500,000
Fixed interest rate					8.00%
Maturity date		31-Jan-13
Accrued interest				$88,000
2012 Notes [Member] | Series B convertible preferred stock [Member]
Debt Instrument [Line Items]
Number of shares issued in exchange for Notes payable				10,195,552
Price per share				$0.45	$0.45

Convertible_Preferred_Stock_Wa2

Convertible Preferred Stock Warrants - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2014
Class Of Stock [Line Items]
Expected volatility		75.00%
Expected term (in years)		1 year 1 month 6 days
Risk-free interest rate		0.13%
Number of securities issued upon exercise of warrants	158,179
Remeasurement of outstanding warrants	173,910
Warrants to purchase convertible preferred stock, exercise price	$5.17
Remeasurement [Member]
Class Of Stock [Line Items]
Expense related to change in fair value	$2,300,000
Reclassified to additional paid in capital	2,800,000
IPO [Member]
Class Of Stock [Line Items]
Warrants to purchase convertible preferred stock, exercise price	$21
2012 Notes [Member]
Class Of Stock [Line Items]
Percentage of shares issuable on conversion of debt as warrants			20.00%
Expected volatility			90.00%
Expected term (in years)			1 year 6 months
Risk-free interest rate			0.21%
Fair value of warrants		474,000	433,000
Warrants term			5 years
Warrants expiration date			12-Oct-17
Series B convertible preferred stock [Member] | 2010 Notes [Member]
Class Of Stock [Line Items]
Number of securities issued upon exercise of warrants			2,000,000
Exercise price per share of warrant exercised			900,000
Convertible preferred stock warrants [Member] | 2010 Notes [Member]
Class Of Stock [Line Items]
Percentage of shares issuable on conversion of debt as warrants			20.00%
Expected volatility			85.00%
Expected term (in years)			1 year 9 months
Risk-free interest rate			0.53%
Fair value of warrants			$233,000

Convertible_Preferred_Stock_Wa3

Convertible Preferred Stock Warrants - Assumptions Used to Value Convertible Preferred Stock Warrants (Detail)	12 Months Ended
	Dec. 31, 2013
Fair Value Assumptions And Methodology For Assets And Liabilities [Abstract]
Expected term (in years)	1 year 1 month 6 days
Expected volatility	75.00%
Risk-free interest rate	0.13%
Dividend yield	0.00%

Commitments_and_Contingencies_1

Commitments and Contingencies - Additional Information (Detail) (USD $)	12 Months Ended			73 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Aug. 31, 2011	Mar. 31, 2014
					sqft	sqft
Loss Contingencies [Line Items]
Rent expense	$520,000	$241,000	$218,000	$1,120,000
Aggregate noncancellable purchase commitments	7,900,000			7,900,000
Other potential commitments	10,000,000			10,000,000
Pending or threatened legal proceedings	0			0
Maximum [Member]
Loss Contingencies [Line Items]
Commitment to make development and sales-related milestone payments	30,000,000			30,000,000
Redwood City, California [Member]
Loss Contingencies [Line Items]
Operating facility lease area					5,740
Operating facility lease term	30 months
Operating facility lease term commencement date	2011-10
Redwood City, California [Member] | Other Assets [Member]
Loss Contingencies [Line Items]
Security deposit paid					55,000
Menlo Park, California [Member]
Loss Contingencies [Line Items]
Operating facility lease area						12,943
Operating facility lease term						39 months
Total obligation under lease	$2,100,000			$2,100,000

Commitments_and_Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Lease Payments Required Under Non-Cancellable Operating Leases (Detail) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Leases [Abstract]
2015	$765
2016	781
2017	529
Total, Future minimum lease payments	$2,075

Schedule_of_Convertible_Prefer

Schedule of Convertible Preferred Stock - Summary of Preferred Stock (Detail) (USD $)	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized	5,000,000	5,000,000	135,816,462
Convertible preferred stock, shares outstanding	0		120,648,174
Convertible preferred stock, Carrying Value			$57,497,000
Series A Preferred Stock [Member]
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized			22,000,000
Convertible preferred stock, shares outstanding			22,000,000
Convertible preferred stock, liquidation preference per share			$0.45
Convertible preferred stock, Carrying Value			9,900,000
Series B Preferred Stock [Member]
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized			46,425,950
Convertible preferred stock, shares outstanding			44,425,953
Convertible preferred stock, liquidation preference per share			$0.45
Convertible preferred stock, Carrying Value			18,631,000
Series C Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized			36,444,444
Convertible preferred stock, shares outstanding			36,444,444
Convertible preferred stock, liquidation preference per share			$0.56
Convertible preferred stock, Carrying Value			19,301,000
Series D-1 Preferred [Member]
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized			17,777,777
Convertible preferred stock, shares outstanding			17,777,777
Convertible preferred stock, liquidation preference per share			$0.56
Convertible preferred stock, Carrying Value			$9,665,000
Series D-2 Preferred [Member]
Temporary Equity [Line Items]
Convertible preferred stock, shares authorized			13,168,291
Convertible preferred stock, liquidation preference per share			$0.76

Convertible_Preferred_Stock_Ad

Convertible Preferred Stock - Additional Information (Detail) (USD $)

1 Months Ended

12 Months Ended

37 Months Ended

73 Months Ended

1 Months Ended

7 Months Ended

1 Months Ended

5 Months Ended

1 Months Ended

12 Months Ended

1 Months Ended

Mar. 31, 2014

Feb. 28, 2011

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2010

Dec. 31, 2011

Dec. 31, 2014

Apr. 30, 2010

Dec. 31, 2009

Dec. 31, 2008

31-May-12

Dec. 31, 2011

Dec. 31, 2012

Jul. 31, 2013

Jan. 31, 2013

Oct. 31, 2013

Feb. 28, 2014

Jan. 31, 2011

Temporary Equity [Line Items]

Carrying value

$57,497,000

Gross proceeds from issuance of stock

1,000,000

Convertible notes converted in to shares

15,876,104

Amount of notes converted

4,600,000

Shares authorized for issuance

5,000,000

5,000,000

135,816,462

5,000,000

Expected volatility

75.00%

Expected term (in years)

1 year 1 month 6 days

Risk-free interest rate

0.13%

Notes receivable exchanged

1,000,000

Accretion term of temporary equity after the issuance date

five years

Accreted during the period

11,000,000

-10,999,000

Preferred stock, conversion ratio

11.5-to-one

Issuance of stock related to IPO

132,100,000

132,137,000

132,137,000

Convertible preferred stock, automatic conversion, Scenario 1 [Member]

Temporary Equity [Line Items]

Issuance of stock related to IPO

50,000,000

Convertible preferred stock, automatic conversion, Scenario 1 [Member] | Minimum [Member]

Temporary Equity [Line Items]

Issuance price per share related to IPO

$17.48

Convertible preferred stock, automatic conversion, Scenario2 [Member]

Temporary Equity [Line Items]

convertible preferred stock, conversion description

the consent of a majority of certain holders of the then outstanding preferred stock, voting together as a class, or, if earlier

Series A Preferred Stock [Member]

Temporary Equity [Line Items]

Carrying value

9,900,000

Shares authorized for issuance

22,000,000

Derivative liability reclassified into preferred stock

12,100,000

Liquidation preference per share

$0.45

Series B Preferred Stock [Member]

Temporary Equity [Line Items]

Carrying value

18,631,000

Shares authorized for issuance

46,425,950

Liquidation preference per share

$0.45

Series C Convertible Preferred Stock [Member]

Temporary Equity [Line Items]

Carrying value

19,301,000

Stock issuance , price per share

$0.56

Shares authorized for issuance

36,444,444

Liquidation preference per share

$0.56

Series D Preferred Stock [Member]

Temporary Equity [Line Items]

Stock issuance , price per share

$0.56

Series D-1 Preferred [Member]

Temporary Equity [Line Items]

Carrying value

9,665,000

Shares authorized for issuance

17,777,777

Liquidation preference per share

$0.56

Series D-2 Preferred [Member]

Temporary Equity [Line Items]

Shares authorized for issuance

13,168,291

Liquidation preference per share

$0.76

Convertible preferred stock [Member]

Temporary Equity [Line Items]

Carrying value

57,497,000

22,722,000

22,722,000

29,647,000

Accreted during the period

10,999,000

Dividends declared

0

Convertible preferred stock [Member] | Series A Preferred Stock [Member]

Temporary Equity [Line Items]

Number of preferred shares agreed to be issued

22,000,000

Number of preferred shares agreed to be issued, share price

$1

Stock issuance, shares issued

3,000,000

8,000,000

Stock issuance , price per share

$1

Derivative liability reclassified into preferred stock

-12,100,000

Noncumulative dividends on preferred stock, per share

$0.04

Preferred Stock, Liquidation Preference Per Share

$0.45

Convertible preferred stock [Member] | Series A Preferred Stock [Member] | Modification [Member]

Temporary Equity [Line Items]

Carrying value

9,900,000

22,000,000

Derivative liability reclassified into preferred stock

-12,100,000

Liquidation preference per share

$0.45

$1

Conversion ratio of convertible preferred stock

$5.17

$11.50

Convertible preferred stock [Member] | Series A Preferred Stock [Member] | Amunix Inc. [Member]

Temporary Equity [Line Items]

Number of preferred shares agreed to be issued

2,500,000

Number of preferred shares agreed to be issued, share price

$1

Stock issuance, shares issued

11,000,000

Convertible preferred stock [Member] | Series A Preferred Stock [Member] | Amunix Inc. [Member] | Options Held [Member]

Temporary Equity [Line Items]

Carrying value

0

Convertible preferred stock [Member] | Series B Preferred Stock [Member]

Temporary Equity [Line Items]

Stock issuance, shares issued

21,805,693

12,861,110

Stock issuance , price per share

$0.45

$0.45

Gross proceeds from issuance of stock

9,800,000

5,800,000

Convertible notes converted in to shares

10,238,444

Noncumulative dividends on preferred stock, per share

$0.04

Preferred Stock, Liquidation Preference Per Share

$0.45

Convertible preferred stock [Member] | Series B Preferred Stock [Member] | Call Option [Member]

Temporary Equity [Line Items]

Expected volatility

71.00%

Expected term (in years)

1 year

Risk-free interest rate

0.28%

Derivative liability, fair value

1,400,000

349,000

349,000

Change in derivative liability recorded in other income (expense), net

1,000,000

88,000

Derivative liability reclassified into preferred stock

260,000

Convertible preferred stock [Member] | Series B Preferred Stock [Member] | Exercise of warrants [Member]

Temporary Equity [Line Items]

Stock issuance, shares issued

2,000,000

Stock issuance, value

900,000

Convertible preferred stock [Member] | Series B Preferred Stock [Member] | Principal [Member]

Temporary Equity [Line Items]

Amount of notes converted

4,500,000

Convertible preferred stock [Member] | Series B Preferred Stock [Member] | Accrued Interest [Member]

Temporary Equity [Line Items]

Amount of notes converted

107,300

Convertible preferred stock [Member] | Series C Convertible Preferred Stock [Member]

Temporary Equity [Line Items]

Stock issuance, shares issued

36,444,444

22,222,222

14,222,222

Stock issuance , price per share

$0.56

$0.56

Gross proceeds from issuance of stock

12,500,000

8,000,000

Stock issuance, value

19,301,000

Noncumulative dividends on preferred stock, per share

$0.05

Preferred Stock, Liquidation Preference Per Share

$0.56

Convertible preferred stock [Member] | Series C Convertible Preferred Stock [Member] | Call Option [Member]

Temporary Equity [Line Items]

Expected volatility

56.00%

Expected term (in years)

6 months

Risk-free interest rate

0.11%

Derivative liability, fair value

990,000

Change in derivative liability recorded in other income (expense), net

864,000

Derivative liability reclassified into preferred stock

126,000

Convertible preferred stock [Member] | Series D Preferred Stock [Member]

Temporary Equity [Line Items]

Stock issuance, shares issued

17,777,777

Stock issuance, value

9,665,000

Shares authorized for issuance

30,946,068

Convertible preferred stock [Member] | Series D Preferred Stock [Member] | Call Option [Member]

Temporary Equity [Line Items]

Expected volatility

60.00%

Expected term (in years)

3 months 18 days

Risk-free interest rate

0.20%

Derivative liability, fair value

21,000

126,000

Change in derivative liability recorded in other income (expense), net

105,000

Convertible preferred stock [Member] | Series D-1 Preferred [Member]

Temporary Equity [Line Items]

Stock issuance, shares issued

17,777,777

Stock issuance , price per share

$0.56

Gross proceeds from issuance of stock

10,000,000

Noncumulative dividends on preferred stock, per share

$0.05

Preferred Stock, Liquidation Preference Per Share

$0.56

Convertible preferred stock [Member] | Series D-2 Preferred [Member]

Temporary Equity [Line Items]

Stock issuance, shares issued

13,168,291

Stock issuance , price per share

$0.76

Gross proceeds from issuance of stock

$10,000,000

Common_Stock_Additional_Inform

Common Stock - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Equity [Abstract]
Common stock, shares authorized	50,000,000	50,000,000	15,652,174
Voting power per share	One
Common stock, dividends declared	$0

Common_Stock_Summary_of_Reserv

Common Stock - Summary of Reserved Common Stock for Future Issuances (Detail)	Dec. 31, 2014	Dec. 31, 2013
Class Of Stock [Line Items]
Common Stock, Capital Shares Reserved for Future Issuance	3,900,281	12,078,240
Convertible preferred stock [Member]
Class Of Stock [Line Items]
Common Stock, Capital Shares Reserved for Future Issuance		10,491,140
Stock Compensation Plan [Member]
Class Of Stock [Line Items]
Common Stock, Capital Shares Reserved for Future Issuance	991,401	9,533
Options to purchase common stock [Member]
Class Of Stock [Line Items]
Common Stock, Capital Shares Reserved for Future Issuance	2,723,366	1,403,655
Restricted Stock Units [Member]
Class Of Stock [Line Items]
Common Stock, Capital Shares Reserved for Future Issuance	185,514
Warrants to purchase convertible preferred stock [Member]
Class Of Stock [Line Items]
Common Stock, Capital Shares Reserved for Future Issuance		173,912

Stock_Based_Awards_Additional_

Stock Based Awards - Additional Information (Detail) (USD $)	12 Months Ended			73 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Feb. 28, 2009
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved	3,900,281	12,078,240		3,900,281
Common stock reserved for future issuance	991,401	9,533	168,571	991,401
Intrinsic value of stock option exercised	$800,000	$0	$0
Unrecognized employee stock-based compensation	20,200,000			20,200,000
Weighted-average remaining vesting period	3 years 1 month 6 days
Additional shares issued	2,531,915	803,590
Voting power per share	One
Recognized compensation expense	4,641,000	215,000	141,000	5,189,000
Weighted Average Remaining Contractual Life, Options outstanding	8 years 9 months 18 days
Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period	4 years
Stock options, weighted-average grant date fair value	$16.90	$1.60	$1.04
Tax benefits realized from options and other share-based payment arrangements	0	0
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Recognized compensation expense	500,000
Approximate compensation expenses	4,300,000
Weighted Average Remaining Contractual Life, Options outstanding	3 years 6 months
2009 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period					4 years
Options vested , description	Options granted generally vest over four years and vest at a rate of 25% upon the first anniversary of the issuance date and 1/36th per month thereafter
2014 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period	4 years
Options expiration period	10 years
Options vested , description	Options granted under the 2014 Plan have a contractual life of ten years and generally vest over four years and vest at a rate of 25% upon the first anniversary of the issuance date and 1/36th per month thereafter.
Number of shares reserved	4,100,000			4,100,000
Percentage of common stock issued and outstanding increase annually	4.50%
End date of automatic annual increase of shares reserved for issuance	1-Jan-24
Common stock reserved for future issuance	991,401			991,401
2014 Plan [Member] | Incentive Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved	12,000,000			12,000,000
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares reserved	150,000			150,000
Percentage of common stock issued and outstanding increase annually	1.00%
End date of automatic annual increase of shares reserved for issuance	1-Jan-24
Additional shares issued	300,000
Voting power per share	Immediately after the grant would own stock possessing 5% or more of the total combined voting power or value of our common stock.
Percentage of stock possessing	5.00%
Rights to purchase stock that remains outstanding	$25,000			$25,000
Offerings of purchase periods	27 months
Maximum employee subscription rate	15.00%			15.00%
Maximum [Member] | 2009 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options expiration period					10 years
Minimum [Member] | 2009 Plan [Member] | Incentive Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Price of options granted, percentage	100.00%
Minimum [Member] | 2009 Plan [Member] | 10% Shareholders [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Price of options granted, percentage	110.00%
Minimum [Member] | 2014 Plan [Member] | Incentive Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Price of options granted, percentage	100.00%
Minimum [Member] | Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Price of options granted, percentage	85.00%

Stock_Based_Awards_Summary_of_

Stock Based Awards - Summary of Stock Options Activity (Detail) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Shares Available for Grant
Shares Available for Grant, Beginning balance	9,533	168,571
Shares Available for Grant, Additional shares authorized	2,531,915	803,590
Shares Available for Grant, Options granted	-1,471,142	-974,459
Shares Available for Grant, Restricted stock units granted	-185,514
Shares Available for Grant, Options cancelled	106,609	11,831
Shares Available for Grant, Ending balance	991,401	9,533
Number of shares
Number of Shares, Beginning balance	1,403,655	462,471
Number of Shares, Options granted	1,471,142	974,459
Number of Shares, Options exercised	-44,822	-21,444
Number of Shares, Options cancelled	-106,609	-11,831
Number of Shares, Ending balance	2,723,366	1,403,655
Number of Shares, Vested and expected to vest	2,651,182
Number of Shares, Exercisable	648,058
Weighted Average Exercise Price
Weighted Average Exercise Price, Beginning balance	$1.90	$1.31
Weighted Average Exercise Price, Options granted	$18.68	$2.16
Weighted Average Exercise Price, Options exercised	$1.32	$1.27
Weighted Average Exercise Price, Options cancelled	$18.43	$1.30
Weighted Average Exercise Price, Ending balance	$10.33	$1.90
Weighted Average Exercise Price, Vested and expected to vest	$10.27
Weighted Average Exercise Price, Exercisable	$3.04
Weighted Average Remaining Contractual Life (in years)
Weighted Average Remaining Contractual Life, Options outstanding	8 years 9 months 18 days
Weighted Average Remaining Contractual Life, Vested and expected to vest	8 years 9 months 18 days
Weighted Average Remaining Contractual Life, Exercisable	7 years 8 months 12 days
Aggregate Intrinsic Value, Options outstanding	$36,006
Aggregate Intrinsic Value, Vested and expected to vest	35,141
Aggregate Intrinsic Value, Exercisable	$12,858

Stock_Based_Awards_Stock_Optio

Stock Based Awards - Stock Options Outstanding and Exercisable under Stock Option Plans (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Number	2,723,366
Options Exercisable and Vested, Outstanding Number	648,058
$1.27 - $1.61 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Lower Limit	$1.27
Range of Exercise Price, Upper Limit	$1.61
Options Outstanding, Number	769,755
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	7 years 6 months
Options Exercisable and Vested, Outstanding Number	471,420
$2.53 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Lower Limit	$2.53
Range of Exercise Price, Upper Limit	$2.53
Options Outstanding, Number	570,927
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	9 years
Options Exercisable and Vested, Outstanding Number	146,484
Options Exercisable and Vested, Weighted Average Remaining Contractual Life (Years)	9 years
$3.34 - $18.66 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Lower Limit	$3.34
Range of Exercise Price, Upper Limit	$18.66
Options Outstanding, Number	576,987
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	9 years 2 months 12 days
Options Exercisable and Vested, Outstanding Number	1,087
Options Exercisable and Vested, Weighted Average Remaining Contractual Life (Years)	9 years 2 months 12 days
$18.73 - $31.96 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Lower Limit	$18.73
Range of Exercise Price, Upper Limit	$31.96
Options Outstanding, Number	795,697
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	9 years 7 months 6 days
Options Exercisable and Vested, Outstanding Number	29,067
Options Exercisable and Vested, Weighted Average Remaining Contractual Life (Years)	9 years 7 months 6 days
$34.00 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Lower Limit	$34
Range of Exercise Price, Upper Limit	$34
Options Outstanding, Number	10,000
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	9 years 6 months
Options Exercisable and Vested, Weighted Average Remaining Contractual Life (Years)	9 years 6 months

Stock_Based_Awards_Summary_of_1

Stock Based Awards - Summary of Fair Value of Employee Stock Options (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Expected volatility	84.90%	89.10%	91.30%
Risk-free interest rate	1.90%	1.70%	1.10%
Dividend yield	0.00%	0.00%	0.00%
Expected life (in years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days

Stock_Based_Awards_Schedule_of

Stock Based Awards - Schedule of Estimated Stock-Based Compensation Expense (Detail) (USD $)	12 Months Ended			73 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$4,641	$215	$141	$5,189
Research and development [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	1,230	124	91	1,599
General and administrative [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$3,411	$91	$50	$3,590

Stock_Based_Awards_Summary_of_2

Stock Based Awards - Summary of Fair Value of Employees Stock Purchase Rights Under ESPP Using Black-Scholes Model with Following Weighted-Average Assumptions (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	84.90%	89.10%	91.30%
Risk-free interest rate	1.90%	1.70%	1.10%
Dividend yield	0.00%	0.00%	0.00%
Expected life (in years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	54.90%
Risk-free interest rate	0.06%
Dividend yield	0.00%
Expected life (in years)	6 months	0 years

Stock_Based_Awards_Summary_of_3

Stock Based Awards - Summary of Restricted Stock Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Granted	185,514
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Granted	185,514
Number of Shares, Unvested ending balance	185,514
Weighted Average Grant Date Fair Value, Granted	26.77
Weighted Average Grant Date Fair Value, Unvested ending balance	26.77

Related_Party_Transactions_Add

Related Party Transactions - Additional Information (Detail) (USD $)	12 Months Ended			73 Months Ended	12 Months Ended	61 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2011	Dec. 31, 2013
Amunix Inc. [Member]
Related Party Transaction [Line Items]
Additional consideration, milestone payments		$30,000,000
Operating expenses from related party transaction	0	0	74,000	7,400,000
Amounts receivable	0			0
Amounts payable	0			0
Amunix Inc. [Member] | Preferred stock [Member]
Related Party Transaction [Line Items]
Ownership Percentage		10.00%				10.00%
Index Ventures [Member] | Preferred stock [Member]
Related Party Transaction [Line Items]
Ownership Percentage		24.00%				24.00%
Mark de Boer [Member]
Related Party Transaction [Line Items]
Consulting fees			0			120,000
Accelerated vesting of shares of common stock					10,185
Additional stock compensation expense related to accelerated vesting					1,000
Diartis [Member]
Related Party Transaction [Line Items]
Operating expenses from related party transaction	0	0	1,400,000	3,900,000
Amounts receivable	0	0		0		0
Third Parties [Member]
Related Party Transaction [Line Items]
Amounts payable	$0	$0		$0		$0

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Contingency [Line Items]
Provision for federal income taxes	$0	$0	$0
Increase in valuation allowance	12,700,000	9,100,000
Unrecognized tax benefits	332,000	287,000	73,000	62,000
Federal [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	85,700,000
Federal [Member] | Research and Development Tax Credits [Member]
Income Tax Contingency [Line Items]
Tax credits	840,000
Tax credit, expiration	Begin to expire in 2029
Federal [Member] | Orphan Drug Credit [Member]
Income Tax Contingency [Line Items]
Tax credits	1,800,000
Tax credit, expiration	Begins to expire in 2033
State [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards	55,100,000
Operating loss carryforwards, expiration	Begin to expire in 2029
State [Member] | Research and Development Tax Credits [Member]
Income Tax Contingency [Line Items]
Tax credits	$698,000

Income_Taxes_Schedule_of_Incom

Income Taxes - Schedule of Income (Loss) Before Income Taxes Attributed to Geographic Locations (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
United States	($49,850)	($18,497)	($13,217)
Foreign	-7,663
Income (loss) before income taxes	($57,513)	($18,497)	($13,217)

Income_Taxes_Summary_of_Statut

Income Taxes - Summary of Statutory Federal Tax Rate to Income or Loss before Taxes (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Federal tax benefit at statutory rate	34.00%	34.00%	34.00%
Change in valuation allowance	-23.00%	-42.00%	-33.00%
Research and development credits	2.00%	9.00%
Non-deductible warrant	-7.00%
Foreign loss not benefitted	-5.00%
Non-deductible expenses and other	-1.00%	-1.00%	-1.00%
Total	0.00%	0.00%	0.00%

Income_Taxes_Significant_Compo

Income Taxes - Significant Components of Net Deferred Tax Assets (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Income Tax Disclosure [Abstract]
Net operating loss carry forwards	$32,143	$21,414
Research and development tax credits	2,776	2,414
Accruals and reserves	1,693	47
Depreciation and amortization	33	89
Total deferred tax assets	36,645	23,964
Less: Valuation allowance	($36,645)	($23,964)

Income_Taxes_Reconciliation_of

Income Taxes - Reconciliation of Amount of Unrecognized Tax Benefits (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Beginning Balance	$287	$73	$62
Increase/(Decrease) of unrecognized tax benefits taken in prior years		31
Increase/(Decrease) of unrecognized tax benefits related to current year		183	11
Decreases based on tax positions taken during a prior period	-102
Increases based on tax positions taken during a current period	147
Ending Balance	$332	$287	$73

Defined_Contribution_Plan_Addi

Defined Contribution Plan - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014
Compensation And Retirement Disclosure [Abstract]
Matching contributions made	$0

Net_Loss_per_Share_Summary_of_

Net Loss per Share - Summary of Computation of Basic and Diluted Net Loss Per Share (Detail) (USD $)	3 Months Ended								12 Months Ended			73 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	Dec. 31, 2014
Earnings Per Share [Abstract]
Net loss attributable to common stockholders - basic and diluted	($15,653)	($13,834)	($8,600)	($44,985)	($5,814)	($5,009)	($3,655)	($4,019)	($83,072)	($18,497)	($13,217)	($136,818)
Weighted-average shares outstanding									18,921,533	450,061	130,718
Less: weighted average shares subject to repurchase										-61	-15,499
Weighted-average shares used to compute basic and diluted net loss per share	24,215,018	24,194,808	24,194,808	2,788,087	1,257,250	250,745	147,713	132,969	18,921,533	450,000	115,219
Basic and diluted net loss per common share	($0.65)	($0.57)	($0.36)	($16.13)	($4.62)	($19.98)	($24.74)	($30.23)	($4.39)	($41.10)	($114.71)

Net_Loss_per_Share_Summary_of_1

Net Loss per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Shares Outstanding (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013		Dec. 31, 2012
Convertible preferred stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding		10,491,140		5,991,757
Warrants to purchase convertible preferred stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding		173,912	[1]	173,912	[1]
Options to purchase common stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding	2,723,366	1,403,655		462,471
Convertible notes [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding				885,024
Restricted stock units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from computation of diluted shares outstanding	185,514
[1]	Assumes exercise of warrants to purchase convertible preferred stock at $5.17 per share.

Net_Loss_per_Share_Summary_of_2

Net Loss per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Shares Outstanding (Parenthetical) (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Warrants to purchase convertible preferred stock, exercise price	$5.17
Warrants to purchase convertible preferred stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Warrants to purchase convertible preferred stock, exercise price		$5.17	$5.17	$5.17

Subsequent_Event_Additional_In

Subsequent Event - Additional Information (Detail) (Subsequent Event [Member], USD $)	1 Months Ended
In Millions, except Share data, unless otherwise specified	Jan. 31, 2015
Subsequent Event [Member]
Subsequent Event [Line Items]
Issuance of stock, shares	4,999,999
Proceeds from issuance of common stock	$80.30

Quarterly_Results_Unaudited_Sc

Quarterly Results (Unaudited) - Schedule of Quarterly Financial Information (Detail) (USD $)	3 Months Ended								12 Months Ended			37 Months Ended	73 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	Dec. 31, 2011	Dec. 31, 2014
Operating expenses
Research and development	$11,602	$10,515	$5,622	$4,869	$3,454	$4,576	$3,326	$3,499	$32,608	$14,855	$10,963		$78,481
General and administrative	4,338	3,577	2,877	2,714	2,405	685	656	682	13,505	4,428	1,936		23,646
Total operating expenses	15,940	14,092	8,499	7,583	5,859	5,261	3,982	4,181	46,113	19,283	12,899		102,127
Loss from operations	-15,940	-14,092	-8,499	-7,583	-5,859	-5,261	-3,982	-4,181	-46,113	-19,283	-12,899		-102,127
Interest income	43	50	40		1				132	1			135
Interest expense								-128		-128	-393		-863
Other income (expense), net	244	208	-141	-11,843	44	252	327	290	-11,532	913	75		-9,502
Net loss and comprehensive loss	-15,653	-13,834	-8,600	-19,426	-5,814	-5,009	-3,655	-4,019	-57,513	-18,497	-13,217	-23,130	-112,357
Deemed dividend related to beneficial conversion feature of convertible preferred stock				-25,559					-25,559				-25,559
Net loss attributable to common stockholders	($15,653)	($13,834)	($8,600)	($44,985)	($5,814)	($5,009)	($3,655)	($4,019)	($83,072)	($18,497)	($13,217)		($136,818)
Net loss per basic and diluted share attributable to common stockholders	($0.65)	($0.57)	($0.36)	($16.13)	($4.62)	($19.98)	($24.74)	($30.23)	($4.39)	($41.10)	($114.71)
Weighted-average common shares used to compute basic and diluted net loss per share	24,215,018	24,194,808	24,194,808	2,788,087	1,257,250	250,745	147,713	132,969	18,921,533	450,000	115,219

Schedule_II_Valuation_and_Qual1

Schedule II - Valuation and Qualifying Accounts (Detail) (Valuation Allowances for Deferred Tax Assets [Member], USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Valuation Allowances for Deferred Tax Assets [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	$23,964	$14,852	$9,681
Additions/charged to expense	12,681	9,112	5,171
Balance at end of period	$36,645	$23,964	$14,852