v3.25.2
Retirement benefits - Assumptions Used in Accounting for Pension Plan (Detail) - Pension
12 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Mar. 31, 2023
Defined Benefit Plan Disclosure [Line Items]      
Discount rate - net periodic benefit cost [1] 7.20% 7.70% 7.90%
Rate of increase in compensation levels of covered employees 7.00% 7.00% 10.00%
Rate of return on plan assets 6.50% 6.50% 6.50%
[1] Weighted average assumptions used to determine both benefit obligations and net periodic benefit cost.