v3.26.1
Available-for-Sale Debt Securities - Summary of Investments in Debt Securities (Detail) - USD ($)
$ in Millions
 12 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Debt Securities, Available-for-Sale [Line Items]    
Amortized cost $ 4,595.2 $ 6,035.4
Gross unrealized gains 0.9 8.8
Gross unrealized losses (13.4) (30.6)
Fair value 4,582.7 6,013.6
U.S. government bonds [Member]    
Debt Securities, Available-for-Sale [Line Items]    
Amortized cost 1,273.1 1,690.9
Gross unrealized gains 0.6 3.5
Gross unrealized losses (0.9) (5.4)
Fair value 1,272.8 1,689.0
UK government bonds [Member]    
Debt Securities, Available-for-Sale [Line Items]    
Amortized cost 1,070.5 1,088.9
Gross unrealized gains 0.1 0.7
Gross unrealized losses (7.2) (7.8)
Fair value 1,063.4 1,081.8
Other foreign bonds [Member]    
Debt Securities, Available-for-Sale [Line Items]    
Amortized cost 1,699.9 2,538.1
Gross unrealized gains 0.2 4.5
Gross unrealized losses (4.6) (15.8)
Fair value 1,695.5 2,526.8
Corporate debt securities [Member]    
Debt Securities, Available-for-Sale [Line Items]    
Amortized cost 551.7 717.5
Gross unrealized gains 0.0 0.1
Gross unrealized losses (0.7) (1.6)
Fair value $ 551.0 $ 716.0