Available-For-Sale Securities (Fair Values And Gross Unrealized Gains And Losses For The Securities Held) (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,566	$ 1,554
Gross Unrealized Gains	245	238
Gross Unrealized Losses	(19)	(12)
Fair Value	1,792	1,780
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	697	685
Gross Unrealized Gains	234	220
Gross Unrealized Losses	(10)	(8)
Fair Value	921	897
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	451	430
Gross Unrealized Gains	7	9
Gross Unrealized Losses	(3)	(1)
Fair Value	455	438
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	388	333
Gross Unrealized Gains	4	9
Gross Unrealized Losses	(6)	(3)
Fair Value	386	339
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	839	763
Gross Unrealized Gains	11	18
Gross Unrealized Losses	(9)	(4)
Fair Value	841	777
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	30	106
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	30	106
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	201	177
Gross Unrealized Gains	12	14
Gross Unrealized Losses	(1)	0
Fair Value	212	191
Rabbi Trust [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	12	12
Gross Unrealized Gains	10	11
Gross Unrealized Losses	0	0
Fair Value	22	23
Rabbi Trust [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	99	89
Gross Unrealized Gains	1	2
Gross Unrealized Losses	0	0
Fair Value	100	91
Rabbi Trust [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	90	74
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(1)	0
Fair Value	90	75
Rabbi Trust [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	189	163
Gross Unrealized Gains	2	3
Gross Unrealized Losses	(1)	0
Fair Value	190	166
Rabbi Trust [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	0	2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	0	2
Rabbi Trust [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 52	$ 45