Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020		Mar. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Research and development expenses	$ 10,920	$ 8,267	$ 6,833	$ 21,090	$ 7,149	$ 21,483	$ 21,502	$ 37,418	$ 47,110	[1]	$ 87,552	[1]
General and administrative expenses	5,133	5,409	5,051	6,468	6,157	10,933	10,622	11,754	22,061	[2]	39,466	[2]
Total operating expenses	16,053	13,676	11,884	27,558	13,306	32,416	32,124	49,172	69,171		127,018
Net loss	$ (16,648)	$ (14,039)	$ (13,884)	$ (28,057)	$ (9,046)	$ (34,296)	$ (33,835)	$ (51,888)	$ (72,628)		$ (129,065)
Net loss per share attributable to common shareholders - basic and diluted	$ (0.54)	$ (0.62)	$ (0.61)	$ (1.23)	$ (0.45)	$ (2.13)	$ (2.24)	$ (3.85)	$ (2.93)		$ (8.02)

[1] (1) Includes total costs (benefit) allocated from certain wholly owned subsidiaries of the Company's affiliate, Roivant Sciences Ltd., of $0 and $(450) for the years ended March 31, 2020 and 2019, respectively.

[2] (2) Includes total costs allocated from certain wholly owned subsidiaries of the Company's affiliate, Roivant Sciences Ltd., of $147 and $2,898 for the years ended March 31, 2020 and 2019, respectively.