Fair Value (Quantitative Information) (Details) (USD $) In Thousands, unless otherwise specified				3 Months Ended						3 Months Ended																		3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Power Exchange Agreements [Member] 2014 to 2019 [Member] Surrogate Facility Pricing [Member]	Mar. 31, 2014 Power Option Agreement [Member] Black Scholes Merton [Member]		Mar. 31, 2014 Power Option Agreement [Member] 2014 [Member] Black Scholes Merton [Member]		Mar. 31, 2014 Power Option Agreement [Member] 2015 [Member] Black Scholes Merton [Member]	Mar. 31, 2014 Power Option Agreement [Member] 2016 [Member] Black Scholes Merton [Member]	Mar. 31, 2014 Power Option Agreement [Member] 2017 [Member] Black Scholes Merton [Member]		Mar. 31, 2014 Minimum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MWh		Mar. 31, 2014 Minimum [Member] Power Option Agreement [Member] Black Scholes Merton [Member] MWh	Mar. 31, 2014 Maximum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MWh		Mar. 31, 2014 Maximum [Member] Power Option Agreement [Member] Black Scholes Merton [Member] MWh	Mar. 31, 2014 Average [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member]		Mar. 31, 2014 Fair Value, Measurements, Recurring [Member] Power Exchange Agreements [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Power Exchange Agreements [Member]	Mar. 31, 2014 Fair Value, Measurements, Recurring [Member] Power Option Agreement [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Power Option Agreement [Member]	Mar. 31, 2014 Fair Value, Measurements, Recurring [Member] Natural Gas Exchange Agreements [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Natural Gas Exchange Agreements [Member]	Mar. 31, 2014 IDAHO	Mar. 31, 2014 WASHINGTON	Mar. 31, 2014 Sales [Member] Minimum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Mar. 31, 2014 Sales [Member] Maximum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Mar. 31, 2014 Purchase [Member] Minimum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Mar. 31, 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	$ (101,890)	$ (89,675)																		$ (13,624)	$ (14,441)	$ (428)	$ (775)	$ (2,418)	$ (1,219)
Operation and Maintenance Charges												30.18	[1]		53.90	[1]		40.93	[1]							40.93	42.44
Escalation Factor			3.00%
Derivative, Price Risk Option Strike Price								69.98	56.53
Fair Value Assumptions, Expected Volatility Rate				20.00%	[2]	31.00%	[2]			18.00%	[2]
Derivative, Forward Price																												4.54	5.23	3.66	4.45
Transaction/Delivery Volumes												396,984		128,278	397,116		286,307											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 calculations 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 0.31 for 2014 to 0.18 in April 2017.