Fair Value Measurements (Schedule Of Unrealized Gains (Losses) Related To Available-For-Sale Investments) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,538		$ 1,498	[1]
Total Unrealized Gains	1,006		905	[1]
Total Unrealized Losses	(8)		(2)	[1]
Total Fair Value	2,536		2,401	[1]
Amount primarily related to deferred taxes on appreciation of investment value	266		240
Money Market Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32		21
Total Fair Value	32		21
U.S. Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	262		331
Total Unrealized Gains	704		618
Total Unrealized Losses	0		0
Total Fair Value	966		949
Non-U.S. Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	205		199
Total Unrealized Gains	188		181
Total Unrealized Losses	(2)		(1)
Total Fair Value	391		379
U.S. Government And Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	831		723
Total Unrealized Gains	67		97
Total Unrealized Losses	(4)		0
Total Fair Value	894		820
Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29		56
Total Unrealized Gains	2		4
Total Unrealized Losses	0		(1)
Total Fair Value	31		59
Other Fixed-Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	166		168
Total Unrealized Gains	2		5
Total Unrealized Losses	(2)		0
Total Fair Value	166		173
Nuclear Decommissioning Trusts [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,525	[1]
Total Unrealized Gains	963	[1]
Total Unrealized Losses	(8)	[1]
Total Fair Value	2,480	[1]
Other Available For Sale Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13
Total Unrealized Gains	43
Total Unrealized Losses	0
Total Fair Value	$ 56

[1] Excludes $266 million and $240 million at June 30, 2013 and December 31, 2012, respectively, primarily related to deferred taxes on appreciation of investment value.