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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans $ (165) $ (155) $ (158)
Past service (cost)/income (36) 4  
Curtailment gains 2 1 5
Settlement gains 5 1  
Net interest on net defined benefit liability (79) (79) (90)
Non-investment expenses paid from the plans (15) (17) (22)
Total defined benefit expense (288) (245) (265)
Current employer service cost for defined contribution and industry-wide plans (244) (255) (257)
Total expense recognised in the income statement (532) $ (500) $ (522)
Pension benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans (156)    
Past service (cost)/income (34)    
Curtailment gains 2    
Settlement gains 5    
Net interest on net defined benefit liability (44)    
Non-investment expenses paid from the plans (15)    
Total defined benefit expense (242)    
Current employer service cost for defined contribution and industry-wide plans (242)    
Total expense recognised in the income statement (484)    
Other Benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans (9)    
Past service (cost)/income (2)    
Net interest on net defined benefit liability (35)    
Total defined benefit expense (46)    
Current employer service cost for defined contribution and industry-wide plans (2)    
Total expense recognised in the income statement $ (48)