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			9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011		Sep. 30, 2012	Sep. 30, 2011
Amount of net pre-tax gain arising during the period included in accumulated other comprehensive loss					$ 2
Fuel and purchased energy expense	6	15		31	61
Revenue		1	[1]		1	[1]
Total net pre-tax loss reclassified into income	6	16		31	62
Net pre-tax gain on commodity derivatives included in accumulated other comprehensive loss	6	16		31	64
Amount reclassified from AOCL to income		$ 1		$ 1

[1] For the three and nine months ended September 30, 2011, $1 million was reclassified from AOCL to income because the forecasted hedged transactions were deemed probable not to occur.