SAN ONOFRE NUCLEAR GENERATING STATION (SONGS) - NUCLEAR DECOMMISSIONING TRUSTS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 701	$ 658
Gross unrealized gains	336	372
Gross unrealized losses	(4)	(4)
Estimated fair value	1,033	1,026
Proceeds from sales	1,314	1,134	$ 577
Gross realized gains	157	111	29
Gross realized losses	(14)	(29)	$ (15)
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	514	396
Gross unrealized gains	10	6
Gross unrealized losses	(3)	(3)
Estimated fair value	521	399
Debt securities issued by the U.S. Treasury and other U.S. government corporations and agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	54	52
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	54	52
Municipal bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	245	203
Gross unrealized gains	7	4
Gross unrealized losses	(2)	(1)
Estimated fair value	250	206
Other securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	215	141
Gross unrealized gains	3	2
Gross unrealized losses	(1)	(2)
Estimated fair value	217	141
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	171	143
Gross unrealized gains	326	366
Gross unrealized losses	(1)	(1)
Estimated fair value	496	508
Cash and cash equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	16	119
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	$ 16	$ 119