Fair Value Measurements (Level 3 Measurements And Sensitivity Analysis) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Jun. 30, 2013		Dec. 31, 2012
Congestion Revenue Rights [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value	$ 72		$ 80
Liabilities, Fair Value	14		16
Fair value measurement Valuation technique	Market approach		Market approach
Fair value measurement Unobservable Input	CRR auction prices		CRR auction prices
Power Purchase Agreements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Fair Value	$ 134		$ 145
Fair value measurement Valuation technique	Discounted cash flow		Discounted cash flow
Fair value measurement Unobservable Input	Forward prices		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	(10.54)	[1]	(9.04)	[1]
Per Mega Watt Hour [Member] | Minimum [Member] | Forward Prices [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Range	10.16	[1]	8.59	[1]
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	7.93	[1]	55.15	[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	56.8	[1]	62.9	[1]

[1] Represents price per megawatt-hour