Available-For-Sale Securities (Fair Values And Gross Unrealized Gains And Losses For The Securities Held) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,497	$ 1,416
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	257	303
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	(15)	(18)
Fair Value	1,739	1,701
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	652	609
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	240	290
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	(9)	(2)
Fair Value	883	897
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	471	438
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	5	3
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	(4)	(12)
Fair Value	472	429
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	344	285
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	12	10
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	(2)	(4)
Fair Value	354	291
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	815	723
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	17	13
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	(6)	(16)
Fair Value	826	720
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	30	84
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	30	84
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	176	172
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	12	10
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	0	(3)
Fair Value	188	179
Rabbi Trust [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	12	14
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	10	9
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	88	109
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	1	0
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	0	(2)
Fair Value	89	107
Rabbi Trust [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	75	46
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	1	1
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	0	(1)
Fair Value	76	46
Rabbi Trust [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	163	155
Available-for-sale Securities, Gross Unrealized Gain Accumulated in Investments	2	1
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	0	(3)
Fair Value	165	153
Rabbi Trust [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	1	3
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	1	3
Rabbi Trust [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 44	$ 39