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Retirement benefits - Summary of Movement in Asset Restriction (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of net defined benefit liability/asset [line items]    
Balance at 1 January $ 3  
Re-measurement(losses)/gains (3) $ 3
Balance at 31 December 0 3
Pension plans [member] | United Kingdom [member]    
Disclosure of net defined benefit liability/asset [line items]    
Balance at 1 January 0  
Re-measurement(losses)/gains 0  
Balance at 31 December 0 0
Pension plans [member] | US and other [member]    
Disclosure of net defined benefit liability/asset [line items]    
Balance at 1 January 3  
Re-measurement(losses)/gains (3) 3
Balance at 31 December 0 3
US post-employment benefits [member]    
Disclosure of net defined benefit liability/asset [line items]    
Balance at 1 January 0  
Re-measurement(losses)/gains 0  
Balance at 31 December $ 0 $ 0