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Summary of Significant Accounting Policies - Narrative (Details)
9 Months Ended
Dec. 31, 2020
USD ($)
segment
Accounting Policies [Abstract]  
Number of operating segments | segment 1
Number of reporting segments | segment 1
Cash, cash equivalents and marketable securities $ 745,800,000
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 86,300,000
Pfizer  
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]  
Outstanding payment from milestones achieved 3,700,000,000
Richter  
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]  
Outstanding payment from milestones achieved $ 137,500,000