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Related party transactions - Chapter 11 Proceedings (Details) - USD ($)
$ in Millions
6 Months Ended
Jul. 06, 2015
Dec. 01, 2008
Oct. 31, 2008
Jun. 30, 2021
West Taurus        
Related Party Transaction [Line Items]        
Remeasurement loss       $ 186
Unwinding of discount debt       54
Payments       $ 31
Purchase price   $ 850    
West Hercules        
Related Party Transaction [Line Items]        
Purchase price     $ 850  
West Linus        
Related Party Transaction [Line Items]        
Purchase price $ 600