Fair Value (Schedule Of Quantitative Information Used For Estimation Of Fair Value Of Level 3) (Detail) (USD $) In Thousands, unless otherwise specified			9 Months Ended						9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 2012 [Member]	Sep. 30, 2012 July 2015 [Member]	Sep. 30, 2012 Power Exchange Agreements [Member]	Sep. 30, 2012 Power Exchange Agreements [Member] 2012 To 2015 [Member] Surrogate Facility Pricing [Member]	Sep. 30, 2012 Power Exchange Agreements [Member] 2016 To 2019 [Member] Surrogate Facility Pricing [Member]	Sep. 30, 2012 Power Option Agreements [Member]	Sep. 30, 2012 Power Option Agreements [Member] Black-Scholes-Merton [Member]		Sep. 30, 2012 Power Option Agreements [Member] 2012 [Member] Black-Scholes-Merton [Member]	Sep. 30, 2012 Power Option Agreements [Member] 2019 [Member] Black-Scholes-Merton [Member]	Sep. 30, 2012 Natural Gas Exchange Agreements [Member]	Sep. 30, 2012 Minimum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MWh		Sep. 30, 2012 Minimum [Member] Power Option Agreements [Member] Black-Scholes-Merton [Member] MWh	Sep. 30, 2012 Minimum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMB	Sep. 30, 2012 Maximum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MWh		Sep. 30, 2012 Maximum [Member] Power Option Agreements [Member] Black-Scholes-Merton [Member] MWh	Sep. 30, 2012 Maximum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMB
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Natural gas exchange agreements	$ (42,070)	$ (98,496)			$ (17,876)			$ (1,585)					$ (3,105)
O&M charges	40.87													30.49	[1]			53.82	[1]
Escalation factor						5.00%	3.00%
Transaction volumes														361,630				379,156
Strike price											41.84	78.21
Delivery volumes																157,517				287,147
Volatility rates, actual			34.00%	24.00%					20.00%	[2]
Forward purchase prices																	3.53				3.69
Forward sales prices																	3.70				4.65
Purchase volumes																	135,000				465,000
Sales volumes																	140,010				310,000

[1] The average O&M charges for 2012 were $40.87 per MWh.

[2] The estimated volatility rate of 0.20 is compared to actual known volatility rates of 0.34 for 2012 to 0.24 in October 2015.