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Summary of Significant Accounting Policies - Narrative (Details)
12 Months Ended
Mar. 31, 2021
USD ($)
segment
Mar. 31, 2020
USD ($)
Mar. 31, 2019
USD ($)
Dec. 26, 2020
Accounting Policies [Abstract]        
Number of operating segments | segment 1      
Number of reporting segments | segment 1      
Cash, cash equivalents and marketable securities $ 684,900,000      
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]        
Advertising costs $ 4,900,000 $ 0 $ 0  
Expected dividend yield 0.00% 0.00% 0.00%  
Minimum        
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]        
Property and equipment useful life 3 years      
Maximum        
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]        
Property and equipment useful life 7 years      
Customer Concentration Risk | Revenues        
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]        
Concentration risk percentage 90.00%      
Customer Concentration Risk | Accounts Receivable        
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]        
Concentration risk percentage 95.00%      
Sumitomo Dainippon Pharma Co., Ltd. | Letter Agreement with Sumitomo Dainippon Pharma, Co., Ltd. | Term Loan | Majority Shareholder        
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]        
Available borrowing capacity $ 41,300,000      
Pfizer        
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]        
Outstanding payment from milestones achieved 3,700,000,000      
Percentage of revenue recognized       100.00%
Richter        
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]        
Outstanding payment from milestones achieved $ 137,500,000