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Post-retirement benefits - Summary of movement in plan assets (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]      
Beginning balance $ (1,730) $ (1,551)  
Settlements 1 (51) $ (5)
Interest on obligation (49) (58) (79)
Employer contributions 201 257 248
Non-investment expenses (16) (14) (15)
Return on plan assets, net of interest on assets (768) (1,033) 527
Assets transferred to defined contribution section (3) (3)  
Currency exchange rate (loss) (62) (20)  
Ending balance (2,273) (1,730) (1,551)
Plan assets      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance 13,923 13,203  
Settlements (7) (587)  
Interest on obligation 335 418  
Contributions by plan participants 22 23  
Employer contributions 201 257  
Benefits paid (778) (862)  
Non-investment expenses (16) (14)  
Return on plan assets, net of interest on assets 768 1,033  
Currency exchange rate (loss) 460 455  
Ending balance 14,905 13,923 $ 13,203
Pension benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (831)    
Settlements 1    
Interest on obligation (22)    
Employer contributions 169    
Non-investment expenses (16)    
Currency exchange rate (loss) (52)    
Ending balance (1,305) (831)  
Pension benefits | Plan assets      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance 13,923    
Settlements (7)    
Interest on obligation 335    
Contributions by plan participants 22    
Employer contributions 169    
Benefits paid (746)    
Non-investment expenses (16)    
Return on plan assets, net of interest on assets 768    
Assets transferred to defined contribution section (3)    
Currency exchange rate (loss) 460    
Ending balance 14,905 13,923  
Other Benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Beginning balance (899)    
Employer contributions 32    
Currency exchange rate (loss) (10)    
Ending balance (968) (899)  
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 32    
Benefits paid (32)    
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) 0    
Ending balance $ 0 $ 0