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Post-retirement benefits - Summary of movement in plan assets (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)  
Settlements (1)   $ (11)
Interest (79) (90) (113)
Contributions by employer 404 464 328
Non-investment expenses (17) (22) (21)
Return on plan assets (net of interest on assets) 894 1,031 79
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)    
Settlements (1)    
Interest (45)    
Contributions by employer 349    
Non-investment expenses (17)    
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)    
Interest (34)    
Contributions by employer 55    
Currency exchange rate (loss)/gain (31)    
Ending balance (998) (926)  
Plan assets [Member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance 13,749 13,642  
Settlements (306)    
Interest 459 492  
Contributions by plan participants 23 23  
Contributions by employer 404 464  
Benefits paid (949) (967)  
Non-investment expenses (17) (22)  
Return on plan assets (net of interest on assets) 894 1,031  
Arrangements divested   (94)  
Currency exchange rate (loss)/gain 1,000 (820)  
Ending balance 15,257 13,749 $ 13,642
Plan assets [Member] | Pension benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance 13,749    
Settlements (306)    
Interest 459    
Contributions by plan participants 23    
Contributions by employer 349    
Benefits paid (894)    
Non-investment expenses (17)    
Return on plan assets (net of interest on assets) 894    
Currency exchange rate (loss)/gain 1,000    
Ending balance 15,257 $ 13,749  
Plan assets [Member] | Other Benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Contributions by employer 55    
Benefits paid $ (55)