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Inventory (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Inventory Disclosure [Abstract]    
Fuel oil $ 74 $ 86
Coal 97 110
Natural gas 28 24
Spare parts 213 233
Total Inventory 412 453
Inventory Write-down $ 3 $ 33