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Employee benefits - Evolution of assets fair value (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Overfunded pension plans | Fair value of assets    
Evolution of assets fair value    
Fair value of plan assets as at beginning of the year $ 4,737 $ 4,828
Interest income 416 406
Employer contributions 27 35
Participant contributions   2
Benefits paid (433) (296)
Return on plan assets (excluding interest income) 757 479
Translation adjustment (200) (717)
Fair value of plan assets as at end of the year 5,304 4,737
Underfunded pension plans    
Evolution of assets fair value    
Fair value of plan assets as at beginning of the year (656)  
Fair value of plan assets as at end of the year (695) (656)
Underfunded pension plans | Fair value of assets    
Evolution of assets fair value    
Fair value of plan assets as at beginning of the year 3,273 3,776
Interest income 123 127
Employer contributions 56 49
Benefits paid (247) (247)
Return on plan assets (excluding interest income) 382 (145)
Translation adjustment 139 (287)
Fair value of plan assets as at end of the year 3,726 3,273
Other benefits    
Evolution of assets fair value    
Fair value of plan assets as at beginning of the year (1,280)  
Fair value of plan assets as at end of the year (1,504) (1,280)
Other benefits | Fair value of assets    
Evolution of assets fair value    
Employer contributions 62 60
Benefits paid $ (62) $ (60)