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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% |  |

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