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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, 2021
Dec. 31, 2020
Dec. 31, 2019
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans $ (167) $ (137) $ (125)
Past service cost (2) (2) 0
Settlement (losses)/gains (3) (1) 51
Net interest on net defined benefit liability (52) (49) (58)
Non-investment expenses paid from the plans (15) (16) (14)
Total defined benefit expense (239) (205) (146)
Current employer service cost for defined contribution and industry-wide plans (315) (264) (238)
Total expense recognised in the income statement (554) $ (469) $ (384)
Pension benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans (159)    
Past service cost 0    
Settlement (losses)/gains (3)    
Net interest on net defined benefit liability (28)    
Non-investment expenses paid from the plans (15)    
Total defined benefit expense (205)    
Current employer service cost for defined contribution and industry-wide plans (313)    
Total expense recognised in the income statement (518)    
Other benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Current employer service cost for defined benefit plans (8)    
Past service cost (2)    
Settlement (losses)/gains 0    
Net interest on net defined benefit liability (24)    
Non-investment expenses paid from the plans 0    
Total defined benefit expense (34)    
Current employer service cost for defined contribution and industry-wide plans (2)    
Total expense recognised in the income statement $ (36)