Fair Value (Quantitative Information) (Details) (USD $) In Thousands, unless otherwise specified				9 Months Ended			9 Months Ended			9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Power Exchange Agreements [Member] 2014 to 2019 [Member] Surrogate Facility Pricing [Member]	Sep. 30, 2013 Power Option Agreement [Member] Black Scholes Merton [Member]		Sep. 30, 2013 Power Option Agreement [Member] 2015 [Member] Black Scholes Merton [Member]	Sep. 30, 2013 Power Option Agreement [Member] 2016 [Member] Black Scholes Merton [Member]	Sep. 30, 2013 Power Option Agreement [Member] 2013 [Member] Black Scholes Merton [Member]	Sep. 30, 2013 Power Option Agreement [Member] 2019 [Member] Black Scholes Merton [Member]	Sep. 30, 2013 Minimum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MWh		Sep. 30, 2013 Minimum [Member] Power Option Agreement [Member] Black Scholes Merton [Member] MWh	Sep. 30, 2013 Minimum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Sep. 30, 2013 Maximum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MWh		Sep. 30, 2013 Maximum [Member] Power Option Agreement [Member] Black Scholes Merton [Member] MWh	Sep. 30, 2013 Maximum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Sep. 30, 2013 Average [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member]		Sep. 30, 2013 Fair Value, Measurements, Recurring [Member] Power Exchange Agreements [Member]	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Power Exchange Agreements [Member]	Sep. 30, 2013 Fair Value, Measurements, Recurring [Member] Power Option Agreement [Member]	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Power Option Agreement [Member]	Sep. 30, 2013 Fair Value, Measurements, Recurring [Member] Natural Gas Exchange Agreements [Member]	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Natural Gas Exchange Agreements [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Gross Liability	$ (115,001)	$ (143,766)																		$ (16,044)	$ (18,692)	$ (761)	$ (1,480)	$ (1,193)	$ (2,379)
Operation and Maintenance Charges										30.18	[1]			53.90	[1]			40.93	[1]
Escalation Factor			3.00%
Derivative, Price Risk Option Strike Price						55.53			69.60
Fair Value Assumptions, Expected Volatility Rate				20.00%	[2]		21.00%	32.00%
Derivative, Forward Price													3.64				4.19
Transaction/Delivery Volumes										365,619		157,517	139,980	394,255		287,147	310,000

[1] The average O&M charges for 2013 were $40.93 per MWh.

[2] The estimated volatility rate of 0.20 is compared to actual quoted volatility rates of 0.32 for 2013 to 0.21 in October 2016.