v3.26.1
Tax - Schedule of Effective Income Tax Rate Reconciliation (Detail) - USD ($)
$ in Millions
12 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Mar. 31, 2024
Income Tax Disclosure [Abstract]      
Income before tax $ 660.4 $ 717.5 $ 656.7
Income Tax Expense (Benefit), Amount [Abstract]      
UK income tax effect 165.1 179.4 164.2
Foreign tax effects 7.2 11.9 4.6
Effect of changes in tax laws or rates enacted in the current period 0.0 0.4 (0.3)
Changes in valuation allowances 1.6 (5.0) 0.0
Nontaxable or nondeductible items (Amount) (11.4) (16.1) (15.1)
Changes in unrecognized tax benefits 0.0 0.0 4.0
Other (0.8) (3.4) (2.2)
Tax (benefit)/expense $ 161.7 $ 167.2 $ 155.2
Income Tax Rate, Percent [Abstract]      
UK income tax effect, rate 25.00% 25.00% 25.00%
Foreign tax effects, rate 1.09% 1.66% 0.70%
Effect of changes in tax laws or rates enacted in the current period, rate 0.00% 0.06% (0.05%)
Changes in valuation allowances, rate 0.24% (0.70%) 0.00%
Nontaxable or nondeductible items, rate (1.71%) (2.24%) (2.30%)
Changes in unrecognized tax benefits, rate 0.00% 0.00% 0.61%
Other (Percent) (0.13%) (0.48%) (0.34%)
Reported income tax (benefit)/expense, rate 24.49% 23.30% 23.62%
Effective tax rate 24.49% 23.30% 23.62%