Related Party Transactions - Narrative (Details)					1 Months Ended	2 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($) shares	Dec. 30, 2019 USD ($)	Dec. 27, 2019 USD ($) day shares	Jun. 04, 2019 USD ($) $ / shares shares	Apr. 30, 2018 USD ($) $ / shares shares	Aug. 31, 2018 USD ($) shares	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Jul. 31, 2018 $ / shares
Related Party Transaction [Line Items]
Debt issued							$ 113,700,000	$ 0	$ 0
Outstanding balance	$ 113,700,000						113,700,000	0
Interest expense (related party)							1,441,000	0	0
Sumitomo Dainippon Pharma, Co., Ltd. | Majority Shareholder
Related Party Transaction [Line Items]
Number of shares owned (in shares) | shares	46,788,604		45,008,604
Ownership percentage	52.10%		50.20%
Ownership threshold for appointment of directors			50.00%
Ownership threshold for voting rights			60.00%
Ownership threshold for right of ownership percentage maintenance			50.00%
Roivant Sciences, Ltd. | Majority Shareholder | Service Agreement
Related Party Transaction [Line Items]
Expenses incurred under agreements							600,000	4,800,000	7,700,000
Roivant Sciences, Ltd. | Majority Shareholder | Share-based Compensation, Expense Allocated to Company
Related Party Transaction [Line Items]
Expenses incurred under agreements							100,000	$ 600,000	$ 1,000,000.0
Private Placement | Roivant Sciences, Ltd. | Majority Shareholder
Related Party Transaction [Line Items]
Number of shares issued (in shares) | shares					1,110,015
Price of shares (in USD per share) | $ / shares					$ 20.27
Net proceeds from sale of shares					$ 22,500,000
Public Equity Offering
Related Party Transaction [Line Items]
Number of shares issued (in shares) | shares				17,424,243		3,533,399
Price of shares (in USD per share) | $ / shares				$ 8.25						$ 22.50
Net proceeds from sale of shares				$ 134,500,000		$ 74,400,000
Public Equity Offering | Majority Shareholder
Related Party Transaction [Line Items]
Number of shares issued (in shares) | shares				2,424,242
Net proceeds from sale of shares				$ 20,000,000.0
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			$ 400,000,000.0
Debt issued		$ 113,700,000
Debt term			5 years
Notice period for prepayment | day			10
Default interest rate			5.00%
Repayment period upon change in control			30 days
Outstanding balance	$ 113,700,000						113,700,000
Available borrowing capacity	$ 286,300,000						286,300,000
Interest expense (related party)							$ 1,400,000
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			3.00%