Fair Value Measurements (Level 3 Measurements And Sensitivity Analysis) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Congestion Revenue Rights [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value	$ 60
Liabilities, Fair Value	(8)
Fair value measurement Valuation technique	Market approach
Fair value measurement Unobservable Input	CRR auction prices
Power Purchase Agreements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value	$ (140)
Fair value measurement Valuation technique	Discounted cash flow
Fair value measurement Unobservable Input	Forward prices
Per Mega Watt Hour [Member] | Minimum [Member] | CRR Auction Prices [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range	(40.74)
Per Mega Watt Hour [Member] | Minimum [Member] | Forward Prices [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range	7.87
Per Mega Watt Hour [Member] | Maximum [Member] | CRR Auction Prices [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range	5.1
Per Mega Watt Hour [Member] | Maximum [Member] | Forward Prices [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range	61.84