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Pension and other post-retirement benefits - Mortality tables and life expectancies (Details 9) - year
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
SAPS S3 [Member]    
Defined benefit plan, Disclosure    
Continuous Mortality Investigation (CMI) model, long-term improvement rate 1.25%  
UK | SAPS S3 [Member] | 65    
Defined benefit plan, Disclosure    
Life expectancy rate at age 65, Male [1] 23.1 23.5
Life expectancy rate at age 65, Female [1] 24.7 25.1
UK | SAPS S3 [Member] | 45    
Defined benefit plan, Disclosure    
Life expectancy rate at age 65, Male [1] 24.3 24.8
Life expectancy rate at age 65, Female [1] 26.1 26.5
US | Pri-2012 [Member] | 65    
Defined benefit plan, Disclosure    
Life expectancy rate at age 65, Male [2] 22.0 20.7
Life expectancy rate at age 65, Female [2] 23.5 22.6
US | Pri-2012 [Member] | 45    
Defined benefit plan, Disclosure    
Life expectancy rate at age 65, Male [2] 23.4 21.9
Life expectancy rate at age 65, Female [2] 24.8 23.7
[1] 102% of Self-Administered Pension Scheme (SAPS) S3 light tables were used, which included CMI projections, with a long-term rate of improvement of 1.25% per annum.
[2] The Private retirement plan 2012 (Pri-2012) mortality tables were used, with projections based on the Social Security Administration's intermediate improvement scale.