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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans $ (125) $ (165) $ (155)
Past service (cost)/income 0 (36) 4
Curtailment gains 0 2 1
Settlement gains 51 5 1
Net interest on net defined benefit liability (58) (79) (79)
Non-investment expenses paid from the plans (14) (15) (17)
Total defined benefit expense (146) (288) (245)
Current employer service cost for defined contribution and industry-wide plans (238) (244) (255)
Total expense recognised in the income statement (384) $ (532) $ (500)
Pension benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans (119)    
Past service (cost)/income 0    
Curtailment gains 0    
Settlement gains 51    
Net interest on net defined benefit liability (22)    
Non-investment expenses paid from the plans (14)    
Total defined benefit expense (104)    
Current employer service cost for defined contribution and industry-wide plans (235)    
Total expense recognised in the income statement (339)    
Other Benefits [member]      
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans (6)    
Past service (cost)/income 0    
Net interest on net defined benefit liability (36)    
Total defined benefit expense (42)    
Current employer service cost for defined contribution and industry-wide plans (3)    
Total expense recognised in the income statement $ (45)