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Post-retirement benefits - Summary of funding policy and contributions to plans (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]      
Contributions to defined benefit plans $ 201 $ 257 $ 248
Contributions to defined contribution plans 261 235 244
Total 462 $ 492 $ 492
Pension benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Contributions to defined benefit plans 169    
Contributions to defined contribution plans 259    
Total 428    
Other Benefits      
Disclosure of net defined benefit liability (asset) [Line Items]      
Contributions to defined benefit plans 32    
Contributions to defined contribution plans 2    
Total $ 34