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Post-retirement benefits - Summary of movement in plan assets (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]      
Beginning balance $ 2,273 $ 1,730  
Employer contributions 464 201 $ 257
Return on plan assets, net of interest on assets (371) (768) (1,033)
Currency exchange rate (loss)/ gain (2) (62)  
Ending balance 1,028 2,273 1,730
Plan assets      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (14,905) (13,923)  
Settlements (383) (7)  
Interest on obligation 276 335  
Contributions by plan participants 22 22  
Employer contributions 464 201  
Benefits paid 837 778  
Non-investment expenses 15 16  
Return on plan assets, net of interest on assets 371 768  
Assets transferred to defined contribution section (4) (3)  
Currency exchange rate (loss)/ gain (99) 460  
Ending balance (14,700) (14,905) $ (13,923)
Pension benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance 1,305    
Employer contributions 425    
Currency exchange rate (loss)/ gain (3)    
Ending balance 194 1,305  
Pension benefits | Plan assets      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (14,905)    
Settlements (383)    
Interest on obligation 276    
Contributions by plan participants 22    
Employer contributions 425    
Benefits paid 798    
Non-investment expenses 15    
Return on plan assets, net of interest on assets 371    
Assets transferred to defined contribution section (4)    
Currency exchange rate (loss)/ gain (99)    
Ending balance (14,700) (14,905)  
Other benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance 968    
Employer contributions 39    
Currency exchange rate (loss)/ gain 1    
Ending balance 834 968  
Other benefits | Plan assets      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance 0    
Settlements 0    
Interest on obligation 0    
Contributions by plan participants 0    
Employer contributions 39    
Benefits paid 39    
Non-investment expenses 0    
Return on plan assets, net of interest on assets 0    
Assets transferred to defined contribution section 0    
Currency exchange rate (loss)/ gain 0    
Ending balance $ 0 $ 0