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Post-retirement benefits - Summary of total expense recognised in the income statement (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of defined benefit plans [Line Items]      
Current employer service cost for defined benefit plans $ (155) $ (158) $ (184)
Past service income 4   144
Curtailment gains 1 5 13
Settlement gains 1   11
Net interest on net defined benefit liability (79) (90) (113)
Non-investment expenses paid from the plans (17) (22) (21)
Total defined benefit expense (245) (265) (150)
Current employer service cost for defined contribution and industry-wide plans (255) (257) (289)
Total expense recognised in the income statement (500) $ (522) $ (439)
Pension benefits [member]      
Disclosure of defined benefit plans [Line Items]      
Current employer service cost for defined benefit plans (146)    
Past service income 1    
Curtailment gains 1    
Settlement gains 1    
Net interest on net defined benefit liability (45)    
Non-investment expenses paid from the plans (17)    
Total defined benefit expense (205)    
Current employer service cost for defined contribution and industry-wide plans (253)    
Total expense recognised in the income statement (458)    
Other Benefits [member]      
Disclosure of defined benefit plans [Line Items]      
Current employer service cost for defined benefit plans (9)    
Past service income 3    
Net interest on net defined benefit liability (34)    
Total defined benefit expense (40)    
Current employer service cost for defined contribution and industry-wide plans (2)    
Total expense recognised in the income statement $ (42)