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Goodwill and Other Intangibles - Schedule of Amortization Expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets $ 107 $ 138 $ 157
Emission Allowances      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 39 71 62
Energy Supply      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 0 1 6
Fuel      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 0 1 1
Customer Relationships      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 32 34 48
Marketing Partnerships      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 9 5 8
Trade Names      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 23 23 23
Other      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets $ 4 $ 3 $ 9