Available-For-Sale Securities (Fair Values And Gross Unrealized Gains And Losses For The Securities Held) (Detail) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		$ 1,674	$ 1,604
Gross Unrealized Gains		348	275
Gross Unrealized Losses		(10)	(21)
Fair Value	[1]	2,012	1,858
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		706	705
Gross Unrealized Gains		331	263
Gross Unrealized Losses		(5)	(11)
Fair Value		1,032	957
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		561	518
Gross Unrealized Gains		10	8
Gross Unrealized Losses		(4)	(6)
Fair Value		567	520
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		352	337
Gross Unrealized Gains		7	4
Gross Unrealized Losses		(1)	(4)
Fair Value		358	337
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		913	855
Gross Unrealized Gains		17	12
Gross Unrealized Losses		(5)	(10)
Fair Value		925	857
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		55	44
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Fair Value		55	44
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		224	209
Gross Unrealized Gains		6	12
Gross Unrealized Losses		(1)	(4)
Fair Value		229	217
Rabbi Trust [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		22	11
Gross Unrealized Gains		1	11
Gross Unrealized Losses		0	0
Fair Value		23	22
Rabbi Trust [Member] | Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		82	105
Gross Unrealized Gains		2	0
Gross Unrealized Losses		0	(2)
Fair Value		84	103
Rabbi Trust [Member] | Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		118	92
Gross Unrealized Gains		3	1
Gross Unrealized Losses		(1)	(2)
Fair Value		120	91
Rabbi Trust [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		200	197
Gross Unrealized Gains		5	1
Gross Unrealized Losses		(1)	(4)
Fair Value		204	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		$ 57	$ 53

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