Retirement Benefit and Other Post-retirement Obligations - Summary of Effect of Members Living One Year More or One Year Less on the Defined Benefit Obligation (Parenthetical) (Detail) - Actuarial Assumption of Longevity [member] |
12 Months Ended |
|---|---|
Dec. 31, 2025 | |
| Disclosure of sensitivity analysis for actuarial assumptions [line items] | |
| Movement in mortality sensitivity analysis | 1 year |
| Movement in mortality sensitivity analysis | 1 year |