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, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total operating expenses		$ 1,657	$ 47,347	$ 41,515	$ 30,311	$ 21,890	$ 22,946	$ 9,056	$ 6,720	$ 17,135	$ 141,063	$ 55,857
Net loss		$ (1,657)	$ (48,253)	$ (41,777)	$ (29,908)	$ (23,317)	$ (21,675)	$ (8,083)	$ (34,712)	$ (18,970)	$ (143,255)	$ (83,440)
Net loss per share attributable to common shareholders - basic and diluted (in USD per share)		$ (0.04)	$ (0.81)	$ (0.70)	$ (0.50)	$ (0.39)	$ (0.37)	$ (0.15)	$ (0.82)	$ (0.47)	$ (2.41)	$ (1.70)
Acquisition of in-process research and development	$ 13,100	$ 0									$ 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
Takeda warrant liability	$ 5,400