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Post-retirement benefits - Summary of movement in present value of obligation (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)  
Current employer service costs 155 158 $ 184
Past service income 4   144
Curtailment gains 1 5 13
Settlements (1)   (11)
Interest (79) (90) (113)
Actuarial losses to equity accounted units (855) (1,120) 548
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)    
Current employer service costs 146    
Past service income 1    
Curtailment gains 1    
Settlements (1)    
Interest (45)    
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)    
Current employer service costs 9    
Past service income 3    
Interest (34)    
Currency exchange rate (loss)/gain (31)    
Ending balance (998) (926)  
Present value of defined benefit obligation [Member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (16,228) (16,123)  
Current employer service costs (155) (158)  
Past service income 4    
Curtailment gains 1 5  
Settlements 307    
Interest (532) (577)  
Contributions by plan participants (23) (23)  
Benefits paid 949 967  
Experience (loss)/gain 7 139  
Actuarial losses to equity accounted units (718) (1,187)  
Changes in demographic assumptions loss (144) (72)  
Arrangements divested 13 86  
Currency exchange rate (loss)/gain (1,126) 715  
Ending balance (17,645) (16,228) $ (16,123)
Present value of defined benefit obligation [Member] | Pension benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (15,302)    
Current employer service costs (146)    
Past service income 1    
Curtailment gains 1    
Settlements 307    
Interest (498)    
Contributions by plan participants (23)    
Benefits paid 894    
Experience (loss)/gain (2)    
Actuarial losses to equity accounted units (678)    
Changes in demographic assumptions loss (119)    
Arrangements divested 13    
Currency exchange rate (loss)/gain (1,095)    
Ending balance (16,647) (15,302)  
Present value of defined benefit obligation [Member] | Other Benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (926)    
Current employer service costs (9)    
Past service income 3    
Interest (34)    
Benefits paid 55    
Experience (loss)/gain 9    
Actuarial losses to equity accounted units (40)    
Changes in demographic assumptions loss (25)    
Currency exchange rate (loss)/gain (31)    
Ending balance $ (998) $ (926)