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Investments Accounted for by the Equity Method and Variable Interest Entities - Other Equity Investments (Details) $ in Millions | Dec. 31, 2017 USD ($) MW | Dec. 31, 2016 USD ($)
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Schedule of Equity Method Investments [Line Items] |  |
Generation capacity (in MW) | MW | 30,000 |
Equity investments in affiliates | $ 1,038 | $ 1,120
Deficit Restoration Obligation | 110 |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |
Current assets | 118 | 87
Net property, plant and equipment | 2,337 | 1,534
Other long-term assets | 658 | 954
Total assets | 3,113 | 2,575
Current liabilities | 96 | 59
Long-term debt | 661 | 442
Other long-term liabilities | 209 | 183
Total liabilities | 966 | 684
Redeemable noncontrolling interests | 78 | 46
Noncontrolling interests | 507 | 529
Net assets less noncontrolling interests | $ 1,562 | $ 1,316
Gladstone Power Station |  |
Schedule of Equity Method Investments [Line Items] |  |
Ownership percentage | 37.50% |
Generation capacity (in MW) | MW | 1,613 |
Equity investments in affiliates | $ 139 |

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