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Acquisitions, Discontinued Operations and Dispositions - Summary of Results (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
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Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Loss on deconsolidation, net of tax |  |  |  |  |  |  |  |  | $ (13) | $ 0 | $ 0
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent | $ 80 | $ (336) | $ 69 | $ (5) | $ (265) | $ 0 | $ (702) | $ (25) | (192) | (992) | 65
Carlsbad Energy Holdings LLC | Discontinued Operations |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 909 | 1,164 | 1,165
Operating costs and expenses |  |  |  |  |  |  |  |  | (661) | (1,114) | (1,023)
Other expenses |  |  |  |  |  |  |  |  | (174) | (288) | (261)
(Loss)/gain from operations of discontinued components, before tax |  |  |  |  |  |  |  |  | 74 | (238) | (119)
Income tax expense |  |  |  |  |  |  |  |  | 4 | 52 | (20)
(Loss)/Income from operations of discontinued components, net of tax |  |  |  |  |  |  |  |  | 70 | (290) | (99)
Loss on deconsolidation, net of tax |  |  |  |  |  |  |  |  | (134) | 0 | 0
California property tax indemnification |  |  |  |  |  |  |  |  | (153) | 0 | 0
Other |  |  |  |  |  |  |  |  | (75) | 0 | 0
Loss on disposal of discontinued components, net of tax |  |  |  |  |  |  |  |  | (362) | 0 | 0
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent |  |  |  |  |  |  |  |  | (292) | (290) | (99)
South Central Portfolio | Discontinued Operations |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 410 | 422 | 467
Operating costs and expenses |  |  |  |  |  |  |  |  | (346) | (335) | (395)
Other income |  |  |  |  |  |  |  |  | 2 | 0 | 0
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent |  |  |  |  |  |  |  |  | 66 | 87 | 72
GenOn | Discontinued Operations |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 0 | 646 | 1,862
Operating costs and expenses |  |  |  |  |  |  |  |  | 0 | (702) | (1,896)
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal |  |  |  |  |  |  |  |  | 0 | 0 | 294
Other expenses |  |  |  |  |  |  |  |  | 0 | (98) | (168)
(Loss)/gain from operations of discontinued components, before tax |  |  |  |  |  |  |  |  | 0 | (154) | 92
Income tax expense |  |  |  |  |  |  |  |  | 0 | 9 | 11
(Loss)/gain from discontinued operations |  |  |  |  |  |  |  |  | 0 | (163) | 81
Interest income - affiliate |  |  |  |  |  |  |  |  | 3 | 8 | 11
(Loss)/Income from operations of discontinued components, net of tax |  |  |  |  |  |  |  |  | 3 | (155) | 92
Pre-tax loss on deconsolidation |  |  |  |  |  |  |  |  | 0 | (208) | 0
Settlement consideration and services credit |  |  |  |  |  |  |  |  | 63 | (289) | 0
Pension and post-retirement liability assumption |  |  |  |  |  |  |  |  | 21 | (131) | 0
Other |  |  |  |  |  |  |  |  | (53) | (6) | 0
Loss on disposal of discontinued components, net of tax |  |  |  |  |  |  |  |  | 31 | (634) | 0
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent |  |  |  |  |  |  |  |  | $ 34 | $ (789) | $ 92

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