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
	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