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Summary of Significant Accounting Policies - Disaggregated Revenue (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
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
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | $ 8,148 |  |
Mark-to-market for economic hedging activities |  |  |  |  |  |  |  |  | 14 | $ 206 | $ (128)
Operating revenues | $ 1,992 | $ 2,960 | $ 2,461 | $ 2,065 | $ 2,154 | $ 2,618 | $ 2,281 | $ 2,021 | 9,478 | 9,074 | 8,915
Derivative revenue |  |  |  |  |  |  |  |  | 31 | 195 | (100)
Contigent lease revenue |  |  |  |  |  |  |  |  | 104 | 253 | 272
Corporate/Eliminations |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | (20) |  |
Operating revenues |  |  |  |  |  |  |  |  | (1,068) | (923) | (1,122)
Energy revenue |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 1,548 |  |
Derivative revenue |  |  |  |  |  |  |  |  | 1,284 |  |
Energy revenue | Corporate/Eliminations |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | (1,129) |  |
Derivative revenue |  |  |  |  |  |  |  |  | (1,117) |  |
Capacity revenue |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 670 |  |
Derivative revenue |  |  |  |  |  |  |  |  | 138 |  |
Capacity revenue | Corporate/Eliminations |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Derivative revenue |  |  |  |  |  |  |  |  | 0 |  |
Retail revenue | Mass customers |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 5,613 |  |
Retail revenue | Mass customers | Corporate/Eliminations |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | (5) |  |
Retail revenue | Business Solutions customers |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 1,492 |  |
Retail revenue | Business Solutions customers | Corporate/Eliminations |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Total retail revenue |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 7,105 |  |
Total retail revenue | Corporate/Eliminations |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | (5) |  |
Derivative revenue |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Mark-to-market for economic hedging activities |  |  |  |  |  |  |  |  | (130) |  |
Derivative revenue |  |  |  |  |  |  |  |  | 1,309 |  |
Derivative revenue | Corporate/Eliminations |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Mark-to-market for economic hedging activities |  |  |  |  |  |  |  |  | 79 |  |
Derivative revenue |  |  |  |  |  |  |  |  | (1,037) |  |
Other revenue |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 285 |  |
Other revenue | Corporate/Eliminations |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | (2) |  |
Total operating revenue |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 9,478 |  |
Total operating revenue | Corporate/Eliminations |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | (1,057) |  |
Lease Revenue |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 21 |  |
Lease Revenue | Corporate/Eliminations |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Other derivative revenue |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative revenue |  |  |  |  |  |  |  |  | 16 |  |
Other derivative revenue | Corporate/Eliminations |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative revenue |  |  |  |  |  |  |  |  | 0 |  |
Retail | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 7,097 |  |
Operating revenues |  |  |  |  |  |  |  |  | 7,103 | 6,369 | 6,330
Retail | Corporate/Eliminations |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | (9) | (4) | (16)
Retail | Energy revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Derivative revenue |  |  |  |  |  |  |  |  | 0 |  |
Retail | Capacity revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Derivative revenue |  |  |  |  |  |  |  |  | 0 |  |
Retail | Retail revenue | Mass customers | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 5,618 |  |
Retail | Retail revenue | Business Solutions customers | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 1,492 |  |
Retail | Total retail revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 7,110 |  |
Retail | Derivative revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Mark-to-market for economic hedging activities |  |  |  |  |  |  |  |  | (7) |  |
Derivative revenue |  |  |  |  |  |  |  |  | (7) |  |
Retail | Other revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Retail | Total operating revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 7,103 |  |
Retail | Lease Revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 13 |  |
Retail | Other derivative revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative revenue |  |  |  |  |  |  |  |  | 0 |  |
Contigent lease revenue |  |  |  |  |  |  |  |  | 17 |  |
Generation | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 1,071 |  |
Operating revenues |  |  |  |  |  |  |  |  | 3,432 | 3,615 | 3,633
Generation | Corporate/Eliminations |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | (1,085) | $ (877) | $ (999)
Generation | Energy revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 2,677 |  |
Derivative revenue |  |  |  |  |  |  |  |  | 2,401 |  |
Generation | Capacity revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 670 |  |
Derivative revenue |  |  |  |  |  |  |  |  | 138 |  |
Generation | Retail revenue | Mass customers | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Generation | Retail revenue | Business Solutions customers | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Generation | Total retail revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Generation | Derivative revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Mark-to-market for economic hedging activities |  |  |  |  |  |  |  |  | (202) |  |
Derivative revenue |  |  |  |  |  |  |  |  | 2,353 |  |
Generation | Other revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 287 |  |
Generation | Total operating revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 3,432 |  |
Generation | Lease Revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 8 |  |
Generation | Other derivative revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative revenue |  |  |  |  |  |  |  |  | 16 |  |
Generation | TEXAS | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | (664) |  |
Generation | TEXAS | Energy revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 1,585 |  |
Derivative revenue |  |  |  |  |  |  |  |  | 2,332 |  |
Generation | TEXAS | Capacity revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 1 |  |
Derivative revenue |  |  |  |  |  |  |  |  | 0 |  |
Generation | TEXAS | Retail revenue | Mass customers | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Generation | TEXAS | Retail revenue | Business Solutions customers | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Generation | TEXAS | Total retail revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Generation | TEXAS | Derivative revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Mark-to-market for economic hedging activities |  |  |  |  |  |  |  |  | (174) |  |
Derivative revenue |  |  |  |  |  |  |  |  | 2,160 |  |
Generation | TEXAS | Other revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 84 |  |
Generation | TEXAS | Total operating revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 1,496 |  |
Generation | TEXAS | Lease Revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Generation | TEXAS | Other derivative revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative revenue |  |  |  |  |  |  |  |  | 2 |  |
Generation | East/West/Other | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 1,735 |  |
Generation | East/West/Other | Energy revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 1,092 |  |
Derivative revenue |  |  |  |  |  |  |  |  | 69 |  |
Contigent lease revenue |  |  |  |  |  |  |  |  | 5 |  |
Generation | East/West/Other | Capacity revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 669 |  |
Derivative revenue |  |  |  |  |  |  |  |  | 138 |  |
Generation | East/West/Other | Retail revenue | Mass customers | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Generation | East/West/Other | Retail revenue | Business Solutions customers | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Generation | East/West/Other | Total retail revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 0 |  |
Generation | East/West/Other | Derivative revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Mark-to-market for economic hedging activities |  |  |  |  |  |  |  |  | (28) |  |
Derivative revenue |  |  |  |  |  |  |  |  | 193 |  |
Generation | East/West/Other | Other revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 203 |  |
Generation | East/West/Other | Total operating revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 1,936 |  |
Generation | East/West/Other | Lease Revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from contract with customer |  |  |  |  |  |  |  |  | 8 |  |
Generation | East/West/Other | Other derivative revenue | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative revenue |  |  |  |  |  |  |  |  | $ 14 |  |

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