Fair Value (Quantitative Information) (Details) (USD $) In Thousands, unless otherwise specified				9 Months Ended						9 Months Ended																		9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Power Exchange Agreements [Member] 2014 to 2019 [Member] Surrogate Facility Pricing [Member]	Sep. 30, 2014 Power Option Agreement [Member] Black Scholes Merton [Member]		Sep. 30, 2014 Power Option Agreement [Member] 2014 [Member] Black Scholes Merton [Member]		Sep. 30, 2014 Power Option Agreement [Member] 2015 [Member] Black Scholes Merton [Member]	Sep. 30, 2014 Power Option Agreement [Member] 2019 [Member] Black Scholes Merton [Member]	Sep. 30, 2014 Power Option Agreement [Member] 2017 [Member] Black Scholes Merton [Member]		Sep. 30, 2014 Minimum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MWh		Sep. 30, 2014 Minimum [Member] Power Option Agreement [Member] Black Scholes Merton [Member] MWh	Sep. 30, 2014 Maximum [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member] MWh		Sep. 30, 2014 Maximum [Member] Power Option Agreement [Member] Black Scholes Merton [Member] MWh	Sep. 30, 2014 Average [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member]		Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Power Exchange Agreements [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Power Exchange Agreements [Member]	Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Power Option Agreement [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Power Option Agreement [Member]	Sep. 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]	Sep. 30, 2014 WASHINGTON Average [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member]	Sep. 30, 2014 IDAHO Average [Member] Power Exchange Agreements [Member] Surrogate Facility Pricing [Member]	Sep. 30, 2014 Sales [Member] Minimum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Sep. 30, 2014 Sales [Member] Maximum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Sep. 30, 2014 Purchase [Member] Minimum [Member] Natural Gas Exchange Agreements [Member] Internally Derived Weighted Average Cost Of Gas [Member] MMBTU	Sep. 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	$ (103,191)	$ (89,675)																		$ (12,854)	$ (14,441)	$ (362)	$ (775)	$ (1,471)	$ (1,219)
Operation and Maintenance Charges												30.66	[1]		55.56	[1]		42.90	[1]							43.11	42.90
Escalation Factor			3.00%
Derivative, Price Risk Option Strike Price								56.2	67.81
Fair Value Assumptions, Expected Volatility Rate				20.00%	[2]	30.00%	[2]			19.00%	[2]
Derivative, Forward Price																												4.19	4.72	3.43	3.68
Transaction/Delivery Volumes												310,103		32,472	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 2014 were $42.90 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 2014 were $43.11 for Washington and $42.90 for Idaho.

[2] The estimated volatility rate of 0.20 is compared to actual quoted volatility rates of 0.30 for 2014 to 0.19 in October 2017.