Available-For-Sale Securities (Fair Values And Gross Unrealized Gains And Losses For The Securities Held) (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,579	$ 1,584
Gross Unrealized Gains	233	196
Gross Unrealized Losses	(15)	(26)
Fair Value	1,797	1,754
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	679	693
Gross Unrealized Gains	198	185
Gross Unrealized Losses	(12)	(13)
Fair Value	865	865
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	503	483
Gross Unrealized Gains	23	8
Gross Unrealized Losses	0	(3)
Fair Value	526	488
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	348	366
Gross Unrealized Gains	12	3
Gross Unrealized Losses	(3)	(10)
Fair Value	357	359
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	851	849
Gross Unrealized Gains	35	11
Gross Unrealized Losses	(3)	(13)
Fair Value	883	847
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	49	42
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	49	42
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	210	204
Gross Unrealized Gains	13	11
Gross Unrealized Losses	(1)	(2)
Fair Value	222	213
Rabbi Trust [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	11	12
Gross Unrealized Gains	10	10
Gross Unrealized Losses	0	0
Fair Value	21	22
Rabbi Trust [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	104	108
Gross Unrealized Gains	2	1
Gross Unrealized Losses	0	(1)
Fair Value	106	108
Rabbi Trust [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	90	82
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(1)	(1)
Fair Value	90	81
Rabbi Trust [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	194	190
Gross Unrealized Gains	3	1
Gross Unrealized Losses	(1)	(2)
Fair Value	196	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	$ 54	$ 52