Subsequent Events (Details) - USD ($) $ in Thousands					1 Months Ended		12 Months Ended
	May 14, 2020	Mar. 31, 2020	Dec. 30, 2019	Dec. 27, 2019	May 14, 2020	Apr. 30, 2020	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Subsequent Event [Line Items]
Debt issued							$ 113,700	$ 0	$ 0
Subsequent Event
Subsequent Event [Line Items]
Proceeds from regulatory milestone payments						$ 10,000
Majority Shareholder | Sumitomo Dainippon Pharma, Co., Ltd.
Subsequent Event [Line Items]
Number of shares owned (in shares)		46,788,604		45,008,604
Ownership percentage		52.10%		50.20%
Majority Shareholder | Sumitomo Dainippon Pharma, Co., Ltd. | Subsequent Event
Subsequent Event [Line Items]
Number of shares issued (in shares)					1,679,868
Number of shares owned (in shares)	48,468,472
Ownership percentage	53.90%
Term Loan | Majority Shareholder | Letter Agreement with Sumitomo Dainippon Pharma, Co., Ltd. | Sumitomo Dainippon Pharma, Co., Ltd.
Subsequent Event [Line Items]
Debt issued			$ 113,700
Available borrowing capacity		$ 286,300					$ 286,300
Term Loan | Majority Shareholder | Letter Agreement with Sumitomo Dainippon Pharma, Co., Ltd. | Sumitomo Dainippon Pharma, Co., Ltd. | Subsequent Event
Subsequent Event [Line Items]
Debt issued						80,000
Available borrowing capacity						$ 206,300