Related Party Transactions (Details)			1 Months Ended	3 Months Ended		6 Months Ended
	Aug. 01, 2020	Dec. 27, 2019 USD ($) director day	Aug. 31, 2020	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) shares	Sep. 30, 2020 USD ($)	Mar. 31, 2021 USD ($)
Related Party Transaction [Line Items]
Amounts due to related parties				$ 334,000		$ 334,000		$ 543,000
Sumitomo Dainippon Pharma Co., Ltd. | Majority Shareholder
Related Party Transaction [Line Items]
Number of shares owned (in shares) | shares				50,041,181		50,041,181
Ownership percentage				53.80%		53.80%
Ownership threshold for appointment of directors		50.00%
Number of independent directors required for audit committee | director		3
Ownership threshold for voting rights		60.00%
Ownership threshold for right of ownership percentage maintenance		50.00%
Amounts due to related parties								100,000
Sumitomo Dainippon Pharma Co., Ltd. | Majority Shareholder | Sumitovant Other Expenses Agreement | SG&A expense
Related Party Transaction [Line Items]
Expenses incurred under agreements					$ 200,000		$ 300,000
Sumitomo Dainippon Pharma Co., Ltd. | Majority Shareholder | Sunovion Pharmaceuticals Inc. Market Access Services Agreement
Related Party Transaction [Line Items]
Agreement term	3 years
Amounts due to related parties				$ 300,000		$ 300,000		$ 400,000
Agreement extension term	1 year
Termination notice period	9 months
MSG | Majority Shareholder | Sunovion Pharmaceuticals Inc. Market Access Services Agreement | SG&A expense
Related Party Transaction [Line Items]
Agreement term			2 years
Expenses incurred under agreements				1,200,000	1,100,000	2,400,000	1,100,000
Term Loan | Letter Agreement with Sumitomo Dainippon Pharma, Co., Ltd. | Sumitomo Dainippon Pharma Co., Ltd. | Majority Shareholder
Related Party Transaction [Line Items]
Maximum borrowing commitment		$ 400,000,000
Facility term		5 years
Notice period for prepayment | day		10
Default interest rate (as a percent)		5.00%
Repayment period upon change in control		30 days
Available borrowing capacity				41,300,000		41,300,000
Outstanding balance				358,700,000		358,700,000
Interest expense				$ 2,900,000	$ 2,100,000	$ 5,800,000	$ 4,300,000
LIBOR | Term Loan | Letter Agreement with Sumitomo Dainippon Pharma, Co., Ltd. | Sumitomo Dainippon Pharma Co., Ltd. | Majority Shareholder
Related Party Transaction [Line Items]
Variable interest rate (as a percent)		3.00%