Available-For-Sale Securities (Fair Values And Gross Unrealized Gains And Losses For The Securities Held) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,569	$ 1,554
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	263	238
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	(11)	(12)
Fair Value	1,821	1,780
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	692	685
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	239	220
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	(8)	(8)
Fair Value	923	897
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	486	430
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	12	9
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	0	(1)
Fair Value	498	438
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	360	333
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	12	9
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	(3)	(3)
Fair Value	369	339
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	846	763
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	24	18
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	(3)	(4)
Fair Value	867	777
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	31	106
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	0	0
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	0	0
Fair Value	31	106
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	200	177
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	14	14
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	0	0
Fair Value	214	191
Rabbi Trust [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	11	12
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	11	11
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	0	0
Fair Value	22	23
Rabbi Trust [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	107	89
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	2	2
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	0	0
Fair Value	109	91
Rabbi Trust [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	79	74
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	1	1
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	0	0
Fair Value	80	75
Rabbi Trust [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	186	163
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	3	3
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	0	0
Fair Value	189	166
Rabbi Trust [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	3	2
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	0	0
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	0	0
Fair Value	3	2
Rabbi Trust [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 53	$ 45