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Other assets - Favorable contracts (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Dec. 31, 2018
Jul. 01, 2018
Dec. 31, 2020
Dec. 31, 2019
Finite-lived Intangible Assets [Roll Forward]        
Impairment of favorable contracts     $ (21) $ 0
Amortization of favorable contracts $ (58) $ 0 (1) (134)
Net carrying amount, end of period     10  
Favorable contracts        
Finite-lived Intangible Assets [Roll Forward]        
Gross carrying amount, beginning of period     287 287
Accumulated amortization, end of period (101)   (256) (254)
Net carrying amount, beginning of period     33 186
Amortization of favorable contracts     (2) (153)
Gross carrying amount, end of period 287   266 287
Accumulated amortization, beginning of period     (254) (101)
Net carrying amount, end of period $ 186   $ 10 $ 33