Fair Value (Quantitative Information) (Details) (USD $) In Thousands, unless otherwise specified				6 Months Ended						6 Months Ended																		6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Power Exchange Agreements [Member] 2014 to 2019 [Member] Surrogate Facility Pricing [Member]	Jun. 30, 2014 Power Option Agreement [Member] Black Scholes Merton [Member]		Jun. 30, 2014 Power Option Agreement [Member] 2014 [Member] Black Scholes Merton [Member]		Jun. 30, 2014 Power Option Agreement [Member] 2015 [Member] Black Scholes Merton [Member]	Jun. 30, 2014 Power Option Agreement [Member] 2016 [Member] Black Scholes Merton [Member]	Jun. 30, 2014 Power Option Agreement [Member] 2017 [Member] Black Scholes Merton [Member]		Jun. 30, 2014 Minimum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MWh		Jun. 30, 2014 Minimum [Member] Power Option Agreement [Member] Black Scholes Merton [Member] MWh	Jun. 30, 2014 Maximum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MWh		Jun. 30, 2014 Maximum [Member] Power Option Agreement [Member] Black Scholes Merton [Member] MWh	Jun. 30, 2014 Average [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member]		Jun. 30, 2014 Fair Value, Measurements, Recurring [Member] Power Exchange Agreements [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Power Exchange Agreements [Member]	Jun. 30, 2014 Fair Value, Measurements, Recurring [Member] Power Option Agreement [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Power Option Agreement [Member]	Jun. 30, 2014 Fair Value, Measurements, Recurring [Member] Natural Gas Exchange Agreements [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Natural Gas Exchange Agreements [Member]	Jun. 30, 2014 WASHINGTON Average [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member]	Jun. 30, 2014 IDAHO Average [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member]	Jun. 30, 2014 Sales [Member] Minimum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Jun. 30, 2014 Sales [Member] Maximum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Jun. 30, 2014 Purchase [Member] Minimum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Jun. 30, 2014 Purchase [Member] Maximum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Gross Liability	$ (105,781)	$ (89,675)																		$ (7,919)	$ (14,441)	$ (605)	$ (775)	$ (2,183)	$ (1,219)
Operation and Maintenance Charges												30.18	[1]		53.90	[1]		40.93	[1]							42.44	40.93
Escalation Factor			3.00%
Derivative, Price Risk Option Strike Price								71.88	58.18
Fair Value Assumptions, Expected Volatility Rate				20.00%	[2]	27.00%	[2]			20.00%	[2]
Derivative, Forward Price																												4.47	5.16	3.57	3.86
Transaction/Delivery Volumes												310,103		110,854	397,116		287,147											279,990	310,000	280,000	310,000

[1] The average O&M charges for the delivery year beginning in November 2013 were $40.93 per MWh. For ratemaking purposes the average O&M charges to be included for recovery in retail rates vary slightly between regulatory jurisdictions. The average O&M charges for the delivery year beginning in 2013 were $42.44 for Washington and $40.93 for Idaho.

[2] The estimated volatility rate of 0.20 is compared to actual quoted volatility rates of 0.27 for 2014 to 0.20 in July 2017.