v3.25.4
Retirement Benefit and Other Post-retirement Obligations - Summary of Fair Value of Plan Assets (Detail)
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Insurance [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 30.00% 31.00%
Insurance [member] | UK Group plan [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 30.00% 31.00%
Insurance [member] | Other plans [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 0.00% 0.00%
Equities [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 19.00% 18.00%
Equities [member] | UK Group plan [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 19.00% 17.00%
Equities [member] | Other plans [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 0.00% 1.00%
Fixed Interest Securities [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 8.00% 9.00%
Fixed Interest Securities [member] | UK Group plan [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 8.00% 7.00%
Fixed Interest Securities [member] | Other plans [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 0.00% 2.00%
Property [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 5.00% 5.00%
Property [member] | UK Group plan [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 5.00% 5.00%
Property [member] | Other plans [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 0.00% 0.00%
Pooled asset investment funds (including LDI) [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 21.00% 22.00%
Pooled asset investment funds (including LDI) [member] | UK Group plan [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 21.00% 22.00%
Pooled asset investment funds (including LDI) [member] | Other plans [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 0.00% 0.00%
Infrastructure [Member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 10.00% 11.00%
Infrastructure [Member] | UK Group plan [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 10.00% 11.00%
Infrastructure [Member] | Other plans [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 0.00% 0.00%
Cash and cash equivalents [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 5.00% 2.00%
Cash and cash equivalents [member] | UK Group plan [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 4.00% 2.00%
Cash and cash equivalents [member] | Other plans [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 1.00% 0.00%
Other [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 2.00% 2.00%
Other [member] | UK Group plan [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 2.00% 2.00%
Other [member] | Other plans [member]    
Disclosure of fair value of plan assets [line items]    
Fair value of plan assets 0.00% 0.00%