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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 ($)
MW
Dec. 31, 2016
USD ($)
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