Condensed Consolidated Statements of Operations - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
License and milestone revenue		$ 0	$ 0	$ 33,333	$ 0
Operating expenses:
Research and development		40,521	50,803	84,707	101,920
General and administrative	[1]	31,316	16,603	54,144	30,755
Total operating expenses		71,837	67,406	138,851	132,675
Loss from operations		(71,837)	(67,406)	(105,518)	(132,675)
Interest expense		0	3,788	0	7,581
Interest expense (related party)		2,115	0	4,299	0
Interest income		(38)	(942)	(146)	(1,708)
Other (income) expense, net		(6,718)	121	(10,287)	(584)
Loss before income taxes		(67,196)	(70,373)	(99,384)	(137,964)
Income tax (benefit) expense		(134)	195	538	508
Net loss		$ (67,062)	$ (70,568)	$ (99,922)	$ (138,472)
Net loss per common share — basic and diluted (in USD per share)		$ (0.75)	$ (0.79)	$ (1.12)	$ (1.68)
Weighted average common shares outstanding — basic and diluted (in shares)		89,744,142	88,798,398	89,523,389	82,667,061
Revenue, Product and Service [Extensible List]		us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
General and administrative	[1]	$ 31,316	$ 16,603	$ 54,144	$ 30,755
Sumitomo Dainippon Pharma Co., Ltd. | Majority Shareholder
Operating expenses:
General and administrative		1,291		1,405
General and administrative		$ 1,291		$ 1,405

[1] Includes $1,291 and $1,405 of expense (inclusive of third-party pass through costs) for the three and six months ended September 30, 2020, respectively, pursuant to the terms of the Company’s agreements with Sumitovant Biopharma Ltd. and Sunovion Pharmaceuticals Inc. (see Note 5).