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Post-retirement benefits - Summary of movement in net defined benefit liability (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance $ (2,542) $ (2,530)  
Amounts recognised in Income (245) (265) $ (150)
Actuarial gains/(losses) on post-retirement benefit plans 6 (90) 619
Contributions by employer 404 464 328
Arrangements divested 13 (8)  
Currency exchange rate (loss)/gain (135) (113)  
Ending balance (2,499) (2,542) $ (2,530)
Pension benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (1,616)    
Amounts recognised in Income (205)    
Actuarial gains/(losses) on post-retirement benefit plans 62    
Contributions by employer 349    
Arrangements divested 13    
Currency exchange rate (loss)/gain (104)    
Ending balance (1,501) (1,616)  
Other Benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (926)    
Amounts recognised in Income (40)    
Actuarial gains/(losses) on post-retirement benefit plans (56)    
Contributions by employer 55    
Currency exchange rate (loss)/gain (31)    
Ending balance $ (998) $ (926)