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Employee benefits - Overfunded pension plans - Measurement of assets at fair value (Details) - Overfunded pension plans - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Measurement of plan assets    
Assets as at beginning of the year $ 6,814  
Assets as at end of the year 5,166 $ 6,814
Level 3    
Measurement of plan assets    
Assets as at beginning of the year 638 673
Return on plan assets 38 35
Assets purchases 129 53
Assets sold during the year (170) (96)
Translation adjustment (143) (27)
Assets as at end of the year 491 638
Level 3 | Private equity funds    
Measurement of plan assets    
Assets as at beginning of the year 157 159
Return on plan assets 18 8
Assets purchases 1 1
Assets sold during the year (15) (4)
Translation adjustment (35) (7)
Assets as at end of the year 126 157
Level 3 | Real estate funds    
Measurement of plan assets    
Assets as at beginning of the year 17 15
Return on plan assets (8)  
Assets purchases 1 2
Translation adjustment (4)  
Assets as at end of the year 5 17
Level 3 | Real estate    
Measurement of plan assets    
Assets as at beginning of the year 323 339
Return on plan assets 9 8
Assets purchases 10 4
Assets sold during the year (14) (13)
Translation adjustment (72) (15)
Assets as at end of the year 255 323
Level 3 | Loans to participants    
Measurement of plan assets    
Assets as at beginning of the year 141 160
Return on plan assets 19 19
Assets purchases 117 46
Assets sold during the year (141) (79)
Translation adjustment (32) (5)
Assets as at end of the year $ 104 $ 141