Fair Value Measurements (Schedule Of Unrealized Gains (Losses) Related To Available-For-Sale Investments) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,533		$ 1,498	[1]
Total Unrealized Gains	1,087		905	[1]
Total Unrealized Losses	(5)		(2)	[1]
Total Fair Value	2,615		2,401	[1]
Amount before deducting deferred taxes primariliy related to appreciation of investment value	292		240
Money Market Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26		21
Total Fair Value	26		21
U.S. Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	267		331
Total Unrealized Gains	753		618
Total Unrealized Losses	(1)		0
Total Fair Value	1,019		949
Non-U.S. Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	205		199
Total Unrealized Gains	230		181
Total Unrealized Losses	0		(1)
Total Fair Value	435		379
U.S. Government And Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	870		723
Total Unrealized Gains	63		97
Total Unrealized Losses	(3)		0
Total Fair Value	930		820
Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24		56
Total Unrealized Gains	2		4
Total Unrealized Losses	0		(1)
Total Fair Value	26		59
Other Fixed-Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	128		168
Total Unrealized Gains	1		5
Total Unrealized Losses	(1)		0
Total Fair Value	128		173
Nuclear Decommissioning Trusts [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,520	[1]
Total Unrealized Gains	1,049	[1]
Total Unrealized Losses	(5)	[1]
Total Fair Value	2,564	[1]
Other Available For Sale Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13
Total Unrealized Gains	38
Total Unrealized Losses	0
Total Fair Value	$ 51

[1] Represents amounts before deducting $292 million and $240 million at September 30, 2013 and December 31, 2012, respectively, of deferred taxes primarily related to appreciation of investment value.