Available-For-Sale Securities (Fair Values And Gross Unrealized Gains And Losses For The Securities Held) (Detail) - USD ($) $ in Millions		Jun. 30, 2017	Dec. 31, 2016
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 1,657	$ 1,604
Gross Unrealized Gains		321	275
Gross Unrealized Losses		(10)	(21)
Fair Value	[1]	1,968	1,858
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		699	705
Gross Unrealized Gains		305	263
Gross Unrealized Losses		(5)	(11)
Fair Value		999	957
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		551	518
Gross Unrealized Gains		10	8
Gross Unrealized Losses		(4)	(6)
Fair Value		557	520
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		356	337
Gross Unrealized Gains		6	4
Gross Unrealized Losses		(1)	(4)
Fair Value		361	337
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		907	855
Gross Unrealized Gains		16	12
Gross Unrealized Losses		(5)	(10)
Fair Value		918	857
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		51	44
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Fair Value		51	44
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		222	209
Gross Unrealized Gains		3	12
Gross Unrealized Losses		(1)	(4)
Fair Value		224	217
Rabbi Trust [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		22	11
Gross Unrealized Gains		0	11
Gross Unrealized Losses		0	0
Fair Value		22	22
Rabbi Trust [Member] | Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		85	105
Gross Unrealized Gains		1	0
Gross Unrealized Losses		(1)	(2)
Fair Value		85	103
Rabbi Trust [Member] | Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		113	92
Gross Unrealized Gains		2	1
Gross Unrealized Losses		0	(2)
Fair Value		115	91
Rabbi Trust [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		198	197
Gross Unrealized Gains		3	1
Gross Unrealized Losses		(1)	(4)
Fair Value		200	194
Rabbi Trust [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		2	1
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Fair Value		2	1
Rabbi Trust [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value		$ 55	$ 53

[1] The NDT available-for-sale securities table excludes cash of $1 million which is part of the NDT Fund.