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Related party transactions - Fair Value Gain/Loss on Convertible Bond (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Dec. 31, 2018
Jul. 01, 2018
Dec. 31, 2020
Dec. 31, 2018
Related Party Transaction [Line Items]        
Fair value (loss) / gain $ (11) $ 0 $ (29) $ 0
Archer | Fair value gain/ (loss) of Archer debt component        
Related Party Transaction [Line Items]        
Fair value (loss) / gain 3 (3) 4 2
Archer | Fair value gain/ (loss) of Archer embedded conversion option        
Related Party Transaction [Line Items]        
Fair value (loss) / gain $ 0 $ (9) $ 3 $ 2