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Employee benefits - Long term compensation plan (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of defined benefit plans [line items]      
Long term compensation plan costs $ 71 $ 39 $ 95
Matching Program      
Disclosure of defined benefit plans [line items]      
Vesting period (in years) 3 years    
Performance Share Unit Program - PSU      
Disclosure of defined benefit plans [line items]      
Vesting period (in years) 4 years    
Award paid during 2nd year (in percentage) 20.00%    
Award paid during 3rd year (in percentage) 30.00%    
Award paid during 4th year (in percentage) 50.00%    
Percentage of performance factor based on total return to shareholder metrics 80    
percentage of performance factor based on environmental, social and governance indicator 20