Fair Value (Quantitative Information) (Details) (USD $) In Thousands, unless otherwise specified					12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Power Exchange Agreements [Member] 2013 to 2015 [Member] Surrogate Facility Pricing [Member]	Dec. 31, 2012 Power Exchange Agreements [Member] 2016 to 2019 [Member] Surrogate Facility Pricing [Member]	Dec. 31, 2012 Power Option Agreements [Member] Black Scholes Merton [Member]		Dec. 31, 2012 Power Option Agreements [Member] 2012 [Member] Black Scholes Merton [Member]	Dec. 31, 2012 Power Option Agreements [Member] 2016 [Member] Black Scholes Merton [Member]	Dec. 31, 2012 Power Option Agreements [Member] 2013 [Member] Black Scholes Merton [Member]	Dec. 31, 2012 Power Option Agreements [Member] 2019 [Member] Black Scholes Merton [Member]	Dec. 31, 2012 Minimum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MMBTU		Dec. 31, 2012 Minimum [Member] Power Option Agreements [Member] Black Scholes Merton [Member] MWh	Dec. 31, 2012 Minimum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Dec. 31, 2012 Maximum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MMBTU		Dec. 31, 2012 Maximum [Member] Power Option Agreements [Member] Black Scholes Merton [Member] MWh	Dec. 31, 2012 Maximum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Power Exchange Agreements [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Power Exchange Agreements [Member]	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Power Option Agreements [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Power Option Agreements [Member]	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Natural Gas Exchange Agreements [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Natural Gas Exchange Agreements [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Liability	$ (143,766)	$ (215,126)																	$ (18,692)	$ (9,910)	$ (1,480)	$ (1,260)	$ (2,379)	$ (1,688)
Operation and Maintenance Charges	40.87										30.49	[1]			53.82	[1]
Escalation Factor			5.00%	3.00%
Transaction Volumes											365,619				379,156
Derivative, Price Risk Option Strike Price									52.61	76.63
Delivery Volumes													128,491				287,147
Fair Value Assumptions, Expected Volatility Rate					20.00%	[2]	33.00%	21.00%
Forward Purchase Prices														3.19				3.38
Forward Sale Prices														3.29				4.46
Purchase Volumes														135,000				465,000
Sales Volumes														140,010				620,000

[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 quoted volatility rates of 0.33 for 2012 to 0.21 in January 2016.