Derivative Instruments And Hedging Activities (Number Of Derivative Contracts By Commodity Type) (Detail)	6 Months Ended
	Jun. 30, 2012 mwh MW Contract
Electric Derivatives [Member]
Derivatives, Fair Value [Line Items]
Number of Contracts	681	[1]
MWhs	16,351,706	[2]
Number of Capacity Contracts	77	[1]
MWs	12,273	[2]
Gas Derivatives [Member]
Derivatives, Fair Value [Line Items]
Number of Contracts	725	[1]
Dths	96,403,140	[2]
Number of Capacity Contracts	1,483	[1]

[1] Qualifying derivative contracts, which have been designated as normal purchases or normal sales contracts, are not reported at fair value under the accounting rules for derivatives and hedging and, therefore, are excluded from the table.

[2] Volumes are reported net of long and short positions.