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Inventory (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Inventory Disclosure [Abstract]      
Fuel oil $ 37 $ 73  
Coal 73 93  
Natural gas 22 21  
Spare parts 195 196  
Total Inventory 327 383  
Inventory [Line Items]      
Inventory lower of cost or market adjustment 29 $ 0 $ 2
Fuel Oil      
Inventory [Line Items]      
Inventory lower of cost or market adjustment $ 29