Fair Value Measurements (Schedule Of Unrealized Gains (Losses) Related To Available-For-Sale Investments) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost
Total Unrealized Gains
Total Unrealized Losses
Total Fair Value
Amount primarily related to deferred taxes on appreciation of investment value	347		324
Money Market Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29		17
Total Fair Value	29		17
Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			5
Total Unrealized Gains			28
Total Unrealized Losses			0
Total Fair Value			33
Other Fixed-Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,133		1,059
Total Unrealized Gains	82		75
Total Unrealized Losses	(3)		(4)
Total Fair Value	1,212		1,130
Nuclear Decommissioning Trusts [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,698	[1]	1,596	[1]
Total Unrealized Gains	1,191	[1]	1,162	[1]
Total Unrealized Losses	(16)	[1]	(13)	[1]
Total Fair Value	2,873	[1]	2,745	[1]
Global equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	536		520
Total Unrealized Gains	1,109		1,087
Total Unrealized Losses	(13)		(9)
Total Fair Value	1,632		1,598
Securities assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,698		1,601
Total Unrealized Gains	1,191		1,190
Total Unrealized Losses	(16)		(13)
Total Fair Value	$ 2,873		$ 2,778

[1] Represents amounts before deducting $347 million and $324 million at March 31, 2015 and December 31, 2014, respectively, primarily related to deferred taxes on appreciation of investment value.