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		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Amount of net pre-tax gain arising during the period included in accumulated other comprehensive loss		$ 3		$ 2
Fuel and purchased energy expense	12	19	25	46
Revenue
Total net pre-tax loss reclassified into income	12	19	25	46
Net pre-tax gain on commodity derivatives included in accumulated other comprehensive loss	$ 12	$ 22	$ 25	$ 48