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Risk management and financial instruments - Realized and Unrealized Gains and Losses (Details) - (Loss)/gain recognized relating to derivative financial instruments - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Derivative Instruments, Gain (Loss) [Line Items]    
Loss/(gain) on derivative financial instruments $ 5 $ 1
Interest rate cap agreement    
Derivative Instruments, Gain (Loss) [Line Items]    
Loss/(gain) on derivative financial instruments 0 (2)
Archer convertible debt instrument    
Derivative Instruments, Gain (Loss) [Line Items]    
Loss/(gain) on derivative financial instruments $ 5 $ 3