Organization - Additional Information (Detail)					12 Months Ended	58 Months Ended
	Sep. 28, 2015 USD ($) $ / shares shares	Mar. 11, 2015 USD ($) $ / shares shares	Feb. 05, 2014 USD ($) $ / shares shares	Aug. 29, 2013 USD ($) $ / shares shares	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Voting rights percentage						100.00%
Shares of common stock sold, value					$ 115,442,747
Initial public offering of common stock | shares	7,935,000		7,063,300
Underwriters' option to purchase shares | shares	1,035,000	661,481	921,300
Common stock price per share | $ / shares	$ 43.50		$ 17.50
Net proceeds of public offering	$ 327,400,000	$ 121,800,000	$ 115,400,000
Takeda Pharmaceutical Company Limited [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Total payments received						$ 29,000,000
Date of termination agreement						Oct. 31, 2014
Private Placement [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares of common stock sold, value				$ 60,000,000
Initial public offering of common stock | shares				18,889,307
Shares of common stock sold, value per share | $ / shares				$ 3.1764
Principal amount of convertible promissory notes converted in to common stock				$ 15,300,000
Accrued interest				$ 800,000
IPO [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Initial public offering of common stock | shares		5,411,481
Common stock price per share | $ / shares		$ 24.00
Oneida Resources Corp. [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Outstanding share of capital stock exchanged				0.5
Company's outstanding capital stock, percentage				100.00%
Outstanding option and outstanding warrant of capital stock exchanged				0.5