v3.26.1
Income taxes - Reconciliation of the U.S federal statutory income tax rate including percent (Details) - USD ($)
$ in Thousands
12 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Amount    
Pre tax loss $ (314,491) $ (246,829)
U.S. Federal statutory tax rate (66,043)  
State and local income taxes, net of federal benefit (2,516)  
Federal R&D credit (15,381)  
Other (788)  
Stock compensation 2,204  
Changes in unrecognized tax benefits 9,537  
Total $ (551) $ 468
Percent    
U.S. Federal statutory tax rate 21.00% 21.00%
State and local income taxes, net of federal benefit 0.80% 0.50%
Change in valuation allowance   (24.30%)
Statutory tax rate difference between U.K. and U.S.   3.60%
Other   (1.00%)
Federal R&D credit 4.90% 1.60%
Other 0.30%  
Stock compensation (0.70%) (1.20%)
Changes in unrecognized tax benefits (3.00%)  
Total 0.20% 0.20%
United Kingdom    
Amount    
Change in valuation allowance $ 63,295  
Statutory tax rate difference between U.K. and U.S. (9,938)  
Other $ (1,186)  
Percent    
Change in valuation allowance (20.10%)  
Statutory tax rate difference between U.K. and U.S. 3.20%  
Other 0.40%  
United States    
Amount    
Change in valuation allowance $ 20,265  
Percent    
Change in valuation allowance (6.40%)