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Summary of Significant Accounting Policies (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Funds Deposited by Counterparties      
Number of months beyond which company can not predict the holding of collateral (in months) 12 months    
Collateral received from GenOn   $ 79  
Accounts Receivable, Net [Abstract]      
Allowance for doubtful accounts $ 28 29  
Project Development Costs and Capitalized Interest      
Amount of interest capitalized 34 30 $ 25
Finite-Lived Intangible Assets, Net [Abstract]      
Intangible assets, accumulated amortization $ 1,818 1,688  
Income Taxes      
Unrecognized tax benefits, more-likely-than-not threshold percentage 50.00%    
Revenue Recognition      
Energy revenues from resales of purchased power $ 187 154 165
Unbilled revenues 376 321 307
Leases [Abstract]      
Contingent rental income 879 912 753
Gross Receipts and Sales Taxes      
Gross Receipts Tax 92 101 110
Cost of Energy for Retail Operations      
Transmission and distribution charges not yet billed 107 90 85
Foreign Currency Translation and Transaction Gains and Losses      
Cumulative translation adjustment (2) (11) (10)
Marketing and Advertising Expense      
Marketing and advertising expense 184 247 309
Advertising Expense 42 $ 53 $ 135
Reorganization Costs      
Severance Costs $ 44    
South Texas Project      
Property, Plant and Equipment      
Ownership Interest (as a percent) 44.00%