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Fair Value Measurements (Quantitative Information About Level 3 Fair Value Measurements) (Details) - Southern California Edison - Level 3 - Congestion revenue rights - Market simulation model and auction prices $ in Millions | 3 Months Ended | 12 Months Ended
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Mar. 31, 2018 USD ($) $ / MWh $ / MMBTU MW | Dec. 31, 2017 USD ($) $ / MWh $ / MMBTU MW
Quantitative Information About Level 3 Measurements [Line Items] |  |
Fair Value, Assets | $ | $ 81 | $ 102
Fair Value, Liabilities | $ | $ 0 | $ 0
Load forecast | Minimum |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |
Fair value inputs, power units (in megawatts) | MW | 5,002 | 5,002
Load forecast | Maximum |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |
Fair value inputs, power units (in megawatts) | MW | 22,970 | 22,970
Power prices | Minimum |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |
Fair value inputs, price level ($ per MWh) | (15.00) | (15.00)
Power prices | Maximum |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |
Fair value inputs, price level ($ per MWh) | 120.00 | 120.00
Gas prices | Minimum |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |
Fair value inputs, price level ($ per MWh) | $ / MMBTU | 2.46 | 2.46
Gas prices | Maximum |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |
Fair value inputs, price level ($ per MWh) | $ / MMBTU | 4.37 | 4.37
CAISO CRR Auction prices | Minimum |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |
Fair value inputs, price level ($ per MWh) | (6.22) | (9.41)
CAISO CRR Auction prices | Maximum |  |
Quantitative Information About Level 3 Measurements [Line Items] |  |
Fair value inputs, price level ($ per MWh) | 8.66 | 8.66

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