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Acquisitions, Discontinued Operations and Dispositions - Summary of Results (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Gain/(loss) on disposal of discontinued operations, net of tax                 $ 0 $ (13) $ 0
Income/(loss) from discontinued operations, net of income tax $ (78) $ (2) $ 13 $ 388 $ 80 $ (336) $ 69 $ (5) 321 (192) (992)
Discontinued Operations | South Central Portfolio                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Operating revenue                 31 410 422
Operating costs and expenses                 (23) (346) (335)
Other income                 0 2 0
(Loss)/gain from operations of discontinued components, before tax                 8 66 87
(Loss)/income on disposal of discontinued operations, net of tax                 20 0 0
Income/(loss) from discontinued operations, net of income tax                 28 66 87
Discontinued Operations | Carlsbad Energy Holdings LLC                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Operating revenue                 19 909 1,164
Operating costs and expenses                 (9) (661) (1,114)
Other expenses                 (5) (174) (288)
(Loss)/gain from operations of discontinued components, before tax                 5 74 (238)
Income tax expense                 0 4 52
Income/(loss) from discontinued operations, net of tax                 5 70 (290)
Gain/(loss) on disposal of discontinued operations, net of tax                 265 (134) 0
Income/(expense) from California property tax indemnification                 22 (153) 0
Income/(expense) from other commitments, indemnification and fees                 4 (75) 0
(Loss)/income on disposal of discontinued operations, net of tax                 291 (362) 0
Income/(loss) from discontinued operations, net of income tax                 296 (292) (290)
Discontinued Operations | GenOn                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Operating revenue                 0 0 646
Operating costs and expenses                 0 0 (702)
Other expenses                 0 0 (98)
(Loss)/gain from operations of discontinued components, before tax                 0 0 (154)
Income tax expense                 0 0 9
(Loss)/gain from discontinued operations                 0 0 (163)
Interest income-affiliate                 0 3 8
Income/(loss) from discontinued operations, net of tax                 0 3 (155)
Gain/(loss) on disposal of discontinued operations, net of tax                 0 0 (208)
Settlement consideration, insurance and services credit                 0 63 (289)
Pension and post-retirement liability assumptions                 0 21 (131)
Income/(expense) from other commitments, indemnification and fees                 (3) (53) (6)
(Loss)/income on disposal of discontinued operations, net of tax                 (3) 31 (634)
Income/(loss) from discontinued operations, net of income tax                 $ (3) $ 34 $ (789)