Available-For-Sale Securities (Fair Values And Gross Unrealized Gains And Losses For The Securities) (Detail) - USD ($) $ in Millions	Dec. 31, 2016		Dec. 31, 2015
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 209		$ 204
Gross Unrealized Gains	12		11
Gross Unrealized Losses	(4)		(2)
Fair Value	217		213
Rabbi Trust [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	11		12
Gross Unrealized Gains	11		10
Gross Unrealized Losses	0		0
Fair Value	22		22
Rabbi Trust [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	105		108
Gross Unrealized Gains	0		1
Gross Unrealized Losses	(2)		(1)
Fair Value	103		108
Rabbi Trust [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	92		82
Gross Unrealized Gains	1		0
Gross Unrealized Losses	(2)		(1)
Fair Value	91		81
Rabbi Trust [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	197		190
Gross Unrealized Gains	1		1
Gross Unrealized Losses	(4)		(2)
Fair Value	194		189
Rabbi Trust [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	1		2
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	1		2
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	1,604		1,584
Gross Unrealized Gains	275		196
Gross Unrealized Losses	(21)		(26)
Fair Value	1,858	[1]	1,754
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	705		693
Gross Unrealized Gains	263		185
Gross Unrealized Losses	(11)		(13)
Fair Value	957		865
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	518		483
Gross Unrealized Gains	8		8
Gross Unrealized Losses	(6)		(3)
Fair Value	520		488
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	337		366
Gross Unrealized Gains	4		3
Gross Unrealized Losses	(4)		(10)
Fair Value	337		359
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	855		849
Gross Unrealized Gains	12		11
Gross Unrealized Losses	(10)		(13)
Fair Value	857		847
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	44		42
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	44		42
Power [Member] | Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 53		$ 52

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