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Other assets - Future amortization of favorable contracts (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]      
Weighted average remaining amortization period 19 years 5 months    
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]      
2021 $ 1    
2022 1    
2023 1    
2024 1    
2025 and thereafter 6    
Total 10    
Favorable contracts      
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]      
Total $ 10 $ 33 $ 186