Fair Value (Quantitative Information) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013		Dec. 31, 2012
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Document Period End Date	Jun. 30, 2013
Gross Liability	$ (123,544)		$ (143,766)
Power Exchange Agreements [Member] | 2013 to 2015 [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Escalation Factor	5.00%
Power Exchange Agreements [Member] | 2016 to 2019 [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Escalation Factor	3.00%
Power Option Agreements [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	20.00%	[1]
Power Option Agreements [Member] | 2014 [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Price Risk Option Strike Price	50.92
Power Option Agreements [Member] | 2016 [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	21.00%
Power Option Agreements [Member] | 2013 [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	28.00%
Power Option Agreements [Member] | 2019 [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Price Risk Option Strike Price	76.54
Minimum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Operation and Maintenance Charges	30.49	[2]
Transaction Volumes	365,619
Minimum [Member] | Power Option Agreements [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Delivery Volumes	128,491
Minimum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Forward Sale Prices	3.55
Sales Volumes	139,980
Maximum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Operation and Maintenance Charges	53.82	[2]
Transaction Volumes	379,156
Maximum [Member] | Power Option Agreements [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Delivery Volumes	287,147
Maximum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Forward Sale Prices	4.08
Sales Volumes	310,000
Average [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Operation and Maintenance Charges	40.87	[2]
Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Gross Liability	(22,179)		(18,692)
Fair Value, Measurements, Recurring [Member] | Power Option Agreements [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Gross Liability	(596)		(1,480)
Fair Value, Measurements, Recurring [Member] | Natural Gas Exchange Agreements [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Gross Liability	$ (1,022)		$ (2,379)

[1] The estimated volatility rate of 0.20 is compared to actual quoted volatility rates of 0.28 for 2013 to 0.21 in July 2016.

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