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Goodwill and Other Intangibles - Schedule of Amortization Expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets $ 207 $ 231 $ 238
Emission Allowances      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 73 66 60
Energy Supply      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 0 7 5
Fuel      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 1 2 2
Customer      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 1 2 2
Customer Relationships      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 35 49 67
Marketing Partnerships      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 5 8 14
Trade Names      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 23 22 23
PPA      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets 62 64 51
Other      
Finite-Lived Intangible Assets [Line Items]      
Amortization of Intangible Assets $ 7 $ 11 $ 14