Available-For-Sale Securities (Fair Values And Gross Unrealized Gains And Losses For The Securities Held) (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,549	$ 1,554
Gross Unrealized Gains	192	238
Gross Unrealized Losses	(26)	(12)
Fair Value	1,715	1,780
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	667	685
Gross Unrealized Gains	176	220
Gross Unrealized Losses	(18)	(8)
Fair Value	825	897
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	445	430
Gross Unrealized Gains	10	9
Gross Unrealized Losses	(1)	(1)
Fair Value	454	438
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	402	333
Gross Unrealized Gains	6	9
Gross Unrealized Losses	(7)	(3)
Fair Value	401	339
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	847	763
Gross Unrealized Gains	16	18
Gross Unrealized Losses	(8)	(4)
Fair Value	855	777
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	35	106
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	35	106
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	204	177
Gross Unrealized Gains	10	14
Gross Unrealized Losses	(3)	0
Fair Value	211	191
Rabbi Trust [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	11	12
Gross Unrealized Gains	9	11
Gross Unrealized Losses	0	0
Fair Value	20	23
Rabbi Trust [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	106	89
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(1)	0
Fair Value	106	91
Rabbi Trust [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	86	74
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(2)	0
Fair Value	84	75
Rabbi Trust [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	192	163
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(3)	0
Fair Value	190	166
Rabbi Trust [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	1	2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	1	2
Rabbi Trust [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 52	$ 45