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Marketable securities - Gross Realized Gains and Losses Related to Marketable Securities (Details) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Dec. 31, 2018
Jul. 01, 2018
Dec. 31, 2020
Dec. 31, 2019
Gain (Loss) on Securities [Line Items]        
Total unrealized gain (loss) on marketable securities $ (64) $ (3) $ (3) $ (46)
Seadrill Partners - Common Units - unrealized loss on marketable securities        
Gain (Loss) on Securities [Line Items]        
Total unrealized gain (loss) on marketable securities (45) (5) (2) (43)
Archer - unrealized (loss)/gain on marketable securities        
Gain (Loss) on Securities [Line Items]        
Total unrealized gain (loss) on marketable securities $ (19) $ 2 $ (1) $ (3)