Significant Accounting Policies - Additional Information (Detail) (USD $)	0 Months Ended	3 Months Ended		20 Months Ended
	Dec. 23, 2014	Mar. 31, 2015 Segment	Mar. 31, 2014	Mar. 31, 2015	Dec. 31, 2014	Dec. 23, 2014
Schedule Of Accounting Policies [Line Items]
Accumulated deficit		$ (411,246,000)		$ (411,246,000)	$ (346,281,000)
Aggregate net proceeds received from initial public offering	279,700,000
Convertible preferred stock converted into number of common stock		59,909,397		59,909,397
Carrying value reclassified to common stock and additional paid in capital		387,700,000
Common stock, shares authorized	495,000,000	495,000,000		495,000,000	495,000,000	495,000,000
Preferred stock, shares authorized	5,000,000	5,000,000		5,000,000	5,000,000	5,000,000
Total investments		3,500,000		3,500,000	3,500,000
Impairment losses				0
Estimated fair value of the convertible preferred stock option			3,200,000
Dividends declared by the board		0
Estimated fair value of total success payment obligation		230,300,000		230,300,000	195,900,000
Success payment expense gain		38,900,000	200,000
Success payment liability balance		123,830,000		123,830,000	84,920,000
Dividend yield		0.00%
Unrecognized tax benefits		0		0	0
Number of operating segment		1
Number of reportable segment		1
Maximum [Member]
Schedule Of Accounting Policies [Line Items]
Original maturities of highly liquid investments		3 months
Convertible Preferred Stock Option [Member]
Schedule Of Accounting Policies [Line Items]
Other expense			600,000
IPO [Member]
Schedule Of Accounting Policies [Line Items]
Common stock issued, share	12,676,354
Issuance of common stock price per share	$ 24.00					24.00
Underwriters option [Member]
Schedule Of Accounting Policies [Line Items]
Common stock issued, share	1,653,437
Stock Price Increased to 10% [Member]
Schedule Of Accounting Policies [Line Items]
Stock price increase/decrease percentage		10.00%
Stock price per share		$ 60.66		$ 60.66
Stock price increase/decrease per share		$ 66.73
Change in success payment expense/gain		12,900,000
Stock Price Decreased to 10% [Member]
Schedule Of Accounting Policies [Line Items]
Stock price increase/decrease percentage		10.00%
Stock price per share		$ 60.66		$ 60.66
Stock price increase/decrease per share		$ 54.59
Change in success payment expense/gain		13,500,000
Stock Price Increased to 35% [Member]
Schedule Of Accounting Policies [Line Items]
Stock price increase/decrease percentage		35.00%
Stock price per share		$ 60.66		$ 60.66
Stock price increase/decrease per share		$ 81.89
Change in success payment expense/gain		42,300,000
Stock Price Decreased to 35% [Member]
Schedule Of Accounting Policies [Line Items]
Success payment expense gain		(10,600,000)
Stock price increase/decrease percentage		35.00%
Stock price per share		$ 60.66		$ 60.66
Stock price increase/decrease per share		$ 39.43
Change in success payment expense/gain		$ 49,500,000