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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans $ (137) $ (125) $ (165)
Past service cost (2) 0 (36)
Curtailment gains 0 0 2
Settlement (losses)/gains (1) 51 5
Net interest on net defined benefit liability (49) (58) (79)
Non-investment expenses paid from the plans (16) (14) (15)
Total defined benefit expense (205) (146) (288)
Current employer service cost for defined contribution and industry-wide plans (264) (238) (244)
Total expense recognised in the income statement (469) $ (384) $ (532)
Pension benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans (131)    
Past service cost (2)    
Curtailment gains 0    
Settlement (losses)/gains (1)    
Net interest on net defined benefit liability (22)    
Non-investment expenses paid from the plans (16)    
Total defined benefit expense (172)    
Current employer service cost for defined contribution and industry-wide plans (262)    
Total expense recognised in the income statement (434)    
Other Benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans (6)    
Past service cost 0    
Curtailment gains 0    
Settlement (losses)/gains 0    
Net interest on net defined benefit liability (27)    
Non-investment expenses paid from the plans 0    
Total defined benefit expense (33)    
Current employer service cost for defined contribution and industry-wide plans (2)    
Total expense recognised in the income statement $ (35)