Fair Value Measurements (Level 3 Measurements And Sensitivity Analysis) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Congestion Revenue Rights [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Fair Value	$ 95		$ 107
Liabilities, Fair Value	28		32
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	$ 87		$ 105
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	(19.16)	[1]	(6.47)	[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	22.54	[1]	23.43	[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	12.04	[1]	12.04	[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	63.45	[1]	51.75	[1]

[1] Represents price per megawatt-hour