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Pension benefits - Change in Pension Plan Assets (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at beginning of year $ 39 $ 33 $ 33
Estimated return 0 1 1
Contribution by employer 6 6 0
Benefits paid (1) (2) (1)
Actuarial gain 0 0 2
Settlement (27) 0 0
Foreign currency translations (1) 1 (2)
Fair value of plan assets at end of year $ 16 $ 39 $ 33