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Retirement benefits - Summary of Estimated Future Benefit Payments (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2018
USD ($)
Years
Dec. 31, 2017
USD ($)
Years
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments $ 66 $ 163
Due within one year [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments 5 15
Between one and five years [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments 23 62
More than 5 years [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments 38 86
Pension plans [member] | United Kingdom [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments $ 19 $ 20
Average duration of obligation (years) | Years 19.5 20.5
Pension plans [member] | United Kingdom [member] | Due within one year [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments $ 0  
Pension plans [member] | United Kingdom [member] | Between one and five years [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments 3 $ 3
Pension plans [member] | United Kingdom [member] | More than 5 years [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments 16 17
Pension plans [member] | United States [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments $ 33 $ 129
Average duration of obligation (years) | Years 9.2 10.3
Pension plans [member] | United States [member] | Due within one year [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments $ 4 $ 14
Pension plans [member] | United States [member] | Between one and five years [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments 14 53
Pension plans [member] | United States [member] | More than 5 years [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments 15 62
US post-employment benefits [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments $ 14 $ 14
Average duration of obligation (years) | Years 9.6 10.4
US post-employment benefits [member] | Due within one year [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments $ 1 $ 1
US post-employment benefits [member] | Between one and five years [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments 6 6
US post-employment benefits [member] | More than 5 years [member]    
Disclosure of net defined benefit liability/asset [line items]    
Estimated future benefit payments $ 7 $ 7