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Marketable securities - Additional Information (Details) - USD ($)
$ in Millions
1 Months Ended 6 Months Ended 12 Months Ended
Apr. 30, 2017
Dec. 31, 2018
Jul. 01, 2018
Dec. 31, 2019
Dec. 31, 2017
Jan. 01, 2018
May 01, 2017
Gain (Loss) on Securities [Line Items]              
Gain on debt extinguishment   $ 0 $ 0 $ (22) $ 19    
Investment in marketable security   57   11      
Archer              
Gain (Loss) on Securities [Line Items]              
Ownership percentage 15.70%            
Gain on debt extinguishment $ 19            
Investment in marketable security 0 $ 12   $ 9      
Archer | Financial Restructuring              
Gain (Loss) on Securities [Line Items]              
Carrying value of loan $ 37           $ 45
Fair value | Archer | Financial Restructuring              
Gain (Loss) on Securities [Line Items]              
Carrying value of loan             $ 56
ASU 2016-01 - Financial Instruments              
Gain (Loss) on Securities [Line Items]              
Adoption of accounting standard update           $ 0  
ASU 2016-01 - Financial Instruments | Accumulated other comprehensive income/(loss)              
Gain (Loss) on Securities [Line Items]              
Adoption of accounting standard update           (31)  
ASU 2016-01 - Financial Instruments | Fair value gains              
Gain (Loss) on Securities [Line Items]              
Adoption of accounting standard update           $ 31