Business Combination and Recapitalization - Additional Information (Detail)				3 Months Ended			6 Months Ended
	Sep. 17, 2020 shares	Dec. 18, 2019 USD ($) tradingDay $ / shares shares	Sep. 29, 2019 shares	Sep. 30, 2020 USD ($) shares	Jun. 30, 2020 shares	[1]	Sep. 30, 2020 USD ($) shares	Mar. 31, 2025 $ / shares shares	Mar. 31, 2023 $ / shares shares	Mar. 31, 2020 shares
Business Acquisition [Line Items]
Business combination consideration | $		$ 420,900,000
Business acquisition price per share (in dollars per share) | $ / shares		$ 10.00
Acquisition related costs | $							$ 2,800,000
Financial advisory fees | $							$ 2,300,000
Earnout shares reserved for issuance (in shares)				0			0			20,000,000
Number of trading days | tradingDay		20
Number of days within a trading period | tradingDay		30
Common stock to potential forfeiture in the event that the milestones are not achieved (in shares)			1,800,000
Vesting of sponsor restricted shares (in shares)	1,800,000
Sponsor
Business Acquisition [Line Items]
Vesting of sponsor restricted shares (in shares)			900,000
Forecast | Milestone Achievement One
Business Acquisition [Line Items]
Volume-weighted average price (in dollars per share) | $ / shares								$ 31.50	$ 17.50
Common stock
Business Acquisition [Line Items]
Number of shares issued in business combination		42,080,376
Earnout shares reserved for issuance (in shares)		20,000,000
Vesting of sponsor restricted shares (in shares)				900,000	900,000
Common stock | Forecast | Milestone Achievement One
Business Acquisition [Line Items]
Earnout shares reserved for issuance (in shares)								10,000,000	10,000,000
Immunovant Sciences Ltd
Business Acquisition [Line Items]
Percentage of business acquisition		100.00%
Number of shares issued in business combination		42,080,376
Immunovant Sciences Ltd | Revision of Prior Period, Adjustment
Business Acquisition [Line Items]
Shares exchange ratio		0.48907%
Immunovant Sciences Ltd | Common stock
Business Acquisition [Line Items]
Earnout shares for issuance (in shares) | $				$ 20,000,000			$ 20,000,000
Roivant Sciences Ltd. (RSL) | Common stock
Business Acquisition [Line Items]
Earnout shares for issuance (in shares) | $				$ 17,547,938			$ 17,547,938
Series A  preferred stock | Roivant Sciences Ltd. (RSL)
Business Acquisition [Line Items]
Number of shares issued in business combination		10,000

[1] Retroactively restated for reverse recapitalization as described in Note 1.