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Description of Business and Liquidity - Additional Information (Detail)
$ in Thousands
6 Months Ended
Dec. 18, 2019
shares
Sep. 30, 2020
USD ($)
segment
Mar. 31, 2020
USD ($)
Business Acquisition [Line Items]      
Number of operating segments | segment   1  
Number of reportable segments | segment   1  
Cash | $   $ 444,372 $ 100,571
Accumulated deficit | $ [1]   $ 138,723 $ 91,226
Common stock      
Business Acquisition [Line Items]      
Number of shares issued in business combination 42,080,376    
Series A  preferred stock | Roivant Sciences Ltd. (RSL)      
Business Acquisition [Line Items]      
Number of shares issued in business combination 10,000    
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 | Sellers      
Business Acquisition [Line Items]      
Ownership percentage 100.00%    
[1] Retroactively restated for the reverse recapitalization as described in Note 1.