Selected Quarterly Data (Unaudited) - Schedule of Quarterly Data (Details) - USD ($) $ / shares in Units, $ in Thousands		2 Months Ended	3 Months Ended				12 Months Ended
	Apr. 29, 2016	Mar. 31, 2016	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total operating expenses		$ 1,657	$ 22,946	$ 9,056	$ 6,720	$ 17,135	$ 55,857
Net loss		$ (1,657)	$ (21,675)	$ (8,083)	$ (34,712)	$ (18,970)	$ (83,440)
Net loss per share attributable to common shareholders - basic and diluted (in USD per share)		$ (0.04)	$ (0.37)	$ (0.15)	$ (0.82)	$ (0.47)	$ (1.70)
Purchase of in-process research and development expense	$ 13,100	$ 0					$ 13,117
Research and development expense, fair value of shares issued	$ 7,700						$ 7,740
Shares issued to Takeda under the Takeda license agreement (in shares)	5,077,001
Warrant liability	$ 5,400