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Employee benefits - Change in benefit obligation (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Overfunded pension plans    
Change in benefit obligation    
Benefit obligation (Plan assets) as at beginning of the year $ 1,298 $ 1,220
Interest costs 74 110
Translation adjustment 1,172 (91)
Benefit obligation (Plan assets) as at end of the year 2,266 1,298
Overfunded pension plans | Present value of actuarial liabilities    
Change in benefit obligation    
Benefit obligation (Plan assets) as at beginning of the year 4,006 3,577
Service costs 7 6
Interest costs 222 305
Benefits paid (368) (433)
Effect of changes in the actuarial assumptions 118 718
Translation adjustment (880) (167)
Benefit obligation (Plan assets) as at end of the year 3,105 4,006
Underfunded pension plans    
Change in benefit obligation    
Benefit obligation (Plan assets) as at beginning of the year 695  
Benefit obligation (Plan assets) as at end of the year 641 695
Underfunded pension plans | Present value of actuarial liabilities    
Change in benefit obligation    
Benefit obligation (Plan assets) as at beginning of the year 4,421 3,929
Service costs 53 55
Interest costs 134 153
Benefits paid (248) (249)
Effect of changes in the actuarial assumptions 271 373
Translation adjustment 1 160
Benefit obligation (Plan assets) as at end of the year 4,632 4,421
Other benefits    
Change in benefit obligation    
Benefit obligation (Plan assets) as at beginning of the year 1,504  
Benefit obligation (Plan assets) as at end of the year 1,733 1,504
Other benefits | Present value of actuarial liabilities    
Change in benefit obligation    
Benefit obligation (Plan assets) as at beginning of the year 1,505 1,280
Service costs 18 10
Interest costs 64 59
Benefits paid (53) (62)
Effect of changes in the actuarial assumptions 243 176
Translation adjustment (44) 42
Benefit obligation (Plan assets) as at end of the year $ 1,733 $ 1,505