Available-For-Sale Securities (Fair Values And Gross Unrealized Gains And Losses For The Securities Held) (Detail) - USD ($) $ in Millions		Mar. 31, 2017	Dec. 31, 2016
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 1,620	$ 1,604
Gross Unrealized Gains		310	275
Gross Unrealized Losses		(17)	(21)
Fair Value	[1]	1,913	1,858
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		717	705
Gross Unrealized Gains		297	263
Gross Unrealized Losses		(7)	(11)
Fair Value		1,007	957
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		515	518
Gross Unrealized Gains		8	8
Gross Unrealized Losses		(6)	(6)
Fair Value		517	520
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		342	337
Gross Unrealized Gains		5	4
Gross Unrealized Losses		(4)	(4)
Fair Value		343	337
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		857	855
Gross Unrealized Gains		13	12
Gross Unrealized Losses		(10)	(10)
Fair Value		860	857
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		46	44
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Fair Value		46	44
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		222	209
Gross Unrealized Gains		1	12
Gross Unrealized Losses		(2)	(4)
Fair Value		221	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		99	105
Gross Unrealized Gains		0	0
Gross Unrealized Losses		(1)	(2)
Fair Value		98	103
Rabbi Trust [Member] | Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		99	92
Gross Unrealized Gains		1	1
Gross Unrealized Losses		(1)	(2)
Fair Value		99	91
Rabbi Trust [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		198	197
Gross Unrealized Gains		1	1
Gross Unrealized Losses		(2)	(4)
Fair Value		197	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.