Derivative Instruments And Hedging Activities (Activity Of Energy Commodity Contracts Designated As Cash Flow Hedges) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Amount of net pre-tax loss arising during the period included in Accumulated Other Comprehensive Loss	$ 0		$ 0
Fuel and Purchased Energy expense	3		7
Revenue		[1]		[1]
Total net pre-tax loss reclassified into income	3		7
Net pre-tax gain on commodity derivatives included in Accumulated Other Comprehensive Loss	3		7
Amount reclassified from AOCL to income	$ 0		$ 0

[1] For the three months ended March 31, 2013 and 2012, no amounts were reclassified from AOCL to income because the forecasted hedged transactions were deemed probable not to occur.