Fair Value (Quantitative Information) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) $ / MWH MMBTU MWh $ / MmBtu	Dec. 31, 2015 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Liability | $	$ (150,381)	$ (210,512)
Commodity Option [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	20.00%
Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Escalation Factor	3.00%
Minimum [Member] | Commodity Option [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	157,517
Minimum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Operation And Maintenance Charges	33.59
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	241,558
Maximum [Member] | Commodity Option [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	285,979
Maximum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Operation And Maintenance Charges	49.15
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	396,984
Average [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions Operation And Maintenance Charges	39.22
Fair Value, Measurements, Recurring [Member] | Commodity Option [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Liability | $	$ (76)
Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Liability | $	(13,449)
Fair Value, Measurements, Recurring [Member] | Natural Gas Exchange Agreements [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Liability | $	$ (5,885)
WASHINGTON | Average [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions Operation And Maintenance Charges	44.33
IDAHO | Average [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions Operation And Maintenance Charges	39.22
Sales [Member] | Minimum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	60,000
Derivative, Forward Price | $ / MmBtu	1.90
Sales [Member] | Maximum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	310,000
Derivative, Forward Price | $ / MmBtu	5.14
Purchase [Member] | Minimum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	115,000
Derivative, Forward Price | $ / MmBtu	1.83
Purchase [Member] | Maximum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	310,000
Derivative, Forward Price | $ / MmBtu	3.06
2017 | Commodity Option [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Expected Volatility Rate	35.00%
2019 | Commodity Option [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Price Risk Option Strike Price	37.83
2018 | Commodity Option [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Price Risk Option Strike Price	54.40
Fair Value Assumptions, Expected Volatility Rate	26.00%