Available-For-Sale Securities (Fair Values And Gross Unrealized Gains And Losses For The Securities Held) (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,578	$ 1,584
Gross Unrealized Gains	219	196
Gross Unrealized Losses	(19)	(26)
Fair Value	1,778	1,754
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	690	693
Gross Unrealized Gains	195	185
Gross Unrealized Losses	(13)	(13)
Fair Value	872	865
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	487	483
Gross Unrealized Gains	15	8
Gross Unrealized Losses	0	(3)
Fair Value	502	488
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	369	366
Gross Unrealized Gains	9	3
Gross Unrealized Losses	(6)	(10)
Fair Value	372	359
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	856	849
Gross Unrealized Gains	24	11
Gross Unrealized Losses	(6)	(13)
Fair Value	874	847
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	32	42
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	32	42
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	208	204
Gross Unrealized Gains	11	11
Gross Unrealized Losses	(2)	(2)
Fair Value	217	213
Rabbi Trust [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	12	12
Gross Unrealized Gains	8	10
Gross Unrealized Losses	0	0
Fair Value	20	22
Rabbi Trust [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	107	108
Gross Unrealized Gains	2	1
Gross Unrealized Losses	0	(1)
Fair Value	109	108
Rabbi Trust [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	84	82
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(2)	(1)
Fair Value	83	81
Rabbi Trust [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	191	190
Gross Unrealized Gains	3	1
Gross Unrealized Losses	(2)	(2)
Fair Value	192	189
Rabbi Trust [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	5	2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	5	2
Rabbi Trust [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 53	$ 52