Fair Value (Quantitative Information) (Details) (USD $) In Thousands, unless otherwise specified				12 Months Ended					12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Power Exchange Agreements [Member] 2016 to 2019 [Member] Surrogate Facility Pricing [Member]	Dec. 31, 2013 Power Option Agreement [Member] Black Scholes Merton [Member]		Dec. 31, 2013 Power Option Agreement [Member] 2014 Black Scholes Merton [Member]	Dec. 31, 2013 Power Option Agreement [Member] 2016 Black Scholes Merton [Member]	Dec. 31, 2013 Power Option Agreement [Member] 2019 [Member] Black Scholes Merton [Member]	Dec. 31, 2013 Minimum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MWh		Dec. 31, 2013 Minimum [Member] Power Option Agreement [Member] Black Scholes Merton [Member] MWh	Dec. 31, 2013 Minimum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Dec. 31, 2013 Maximum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MWh		Dec. 31, 2013 Maximum [Member] Power Option Agreement [Member] Black Scholes Merton [Member] MWh	Dec. 31, 2013 Maximum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Power Exchange Agreements [Member]	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Power Exchange Agreements [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Power Option Agreement [Member]	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Power Option Agreement [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Natural Gas Exchange Agreements [Member]	Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Natural Gas Exchange Agreements [Member]	Dec. 31, 2013 WASHINGTON	Dec. 31, 2013 IDAHO
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Liability	$ (89,675)	$ (143,766)															$ (14,441)	$ (18,692)	$ (775)	$ (1,480)	$ (1,219)	$ (2,379)
Operation and Maintenance Charges	40.93								30.18	[1]			53.90	[1]									42.44	40.93
Escalation Factor			3.00%
Derivative, Price Risk Option Strike Price							55.62	65.31
Derivative, Nonmonetary Notional Amount, Energy Measure									365,619		128,491	150,000	379,156		287,147	310,000
Fair Value Assumptions, Expected Volatility Rate				20.00%	[2]	31.00%	20.00%
Derivative, Forward Price												3.98				4.57

[1] The average O&M charges for the delivery year beginning in November 2013 were $40.93 per MWh. For rate-making purposes the average O&M calculations vary slightly between regulatory jurisdictions. For Washington, the average O&M charges were $42.44 and the average O&M charges for Idaho were $40.93 for the delivery year beginning in 2013.

[2] The estimated volatility rate of 0.20 is compared to actual quoted volatility rates of 0.31 for 2014 to 0.20 in October 2016.