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Retirement benefits - Defined Benefit Plans - Life Expectancy Assumptions at Retirement Age (Detail) - Plan
12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
United Kingdom | Current pensioners at 65 - male      
Disclosure Of Forecast Life Expectancies [line items]      
Actuarial assumption of life expectancy after retirement 24 years 23 years 23 years
United Kingdom | Current pensioners at 65 - female      
Disclosure Of Forecast Life Expectancies [line items]      
Actuarial assumption of life expectancy after retirement 26 years 25 years 25 years
United Kingdom | Future pensioners at 65 - male      
Disclosure Of Forecast Life Expectancies [line items]      
Actuarial assumption of life expectancy after retirement 25 years 23 years 23 years
United Kingdom | Future pensioners at 65 - female      
Disclosure Of Forecast Life Expectancies [line items]      
Actuarial assumption of life expectancy after retirement 27 years 25 years 25 years
United States | Current pensioners at 65 - male      
Disclosure Of Forecast Life Expectancies [line items]      
Actuarial assumption of life expectancy after retirement 22 years 22 years 22 years
United States | Current pensioners at 65 - female      
Disclosure Of Forecast Life Expectancies [line items]      
Actuarial assumption of life expectancy after retirement 24 years 23 years 23 years
United States | Future pensioners at 65 - male      
Disclosure Of Forecast Life Expectancies [line items]      
Actuarial assumption of life expectancy after retirement 23 years 23 years 23 years
United States | Future pensioners at 65 - female      
Disclosure Of Forecast Life Expectancies [line items]      
Actuarial assumption of life expectancy after retirement 25 years 25 years 25 years