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Stockholders' equity - Treasury shares (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Stockholders' equity    
Assignment and transfer of shares $ 7 $ 14
Treasury shares    
Stockholders' equity    
Number of treasury shares re-issued as compensation 890,482 1,628,485
Assignment and transfer of shares $ 7 $ 14