Derivative Instruments And Hedging Activities (Activity Of Energy Commodity Contracts Designated As Cash Flow Hedges) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Derivatives And Hedging Activities [Line Items]
Amount of net pre-tax loss arising during the period included in accumulated other comprehensive loss					$ (100)
Fuel and purchased energy expense	38		80		135
Revenue	1	[1]	1	[1]
Total net pre-tax loss reclassified into income	39		81		135
Net pre-tax gain on commodity derivatives included in other comprehensive loss	39		81		35
Amount reclassified from AOCL to income	$ 1		$ 1

[1] Included in the above table is a loss of $1 million for the years ended December 31, 2012 and 2011, respectively, which was reclassified from AOCL to income because the forecasted hedged transactions were deemed probable not to occur.