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Fair Value (Quantitative Information) (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Power Exchange Agreements [Member] | 2014 to 2019 [Member] | Surrogate Facility Pricing [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Escalation Factor 3.00%ava_EscalationFactor
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Power Option Agreement [Member] | Black Scholes Merton [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Assumptions, Expected Volatility Rate 20.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate
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Power Option Agreement [Member] | 2015 [Member] | Black Scholes Merton [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Price Risk Option Strike Price 37.87us-gaap_DerivativePriceRiskOptionStrikePrice
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Fair Value Assumptions, Expected Volatility Rate 38.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate
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Power Option Agreement [Member] | 2019 [Member] | Black Scholes Merton [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Price Risk Option Strike Price 56.46us-gaap_DerivativePriceRiskOptionStrikePrice
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Power Option Agreement [Member] | 2018 [Member] | Black Scholes Merton [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Assumptions, Expected Volatility Rate 20.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate
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Minimum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Operation and Maintenance Charges 30.66ava_OperationAndMaintenanceCharges
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Transaction/Delivery Volumes 396,984us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
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Minimum [Member] | Power Option Agreement [Member] | Black Scholes Merton [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Transaction/Delivery Volumes 157,517us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
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Maximum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Operation and Maintenance Charges 55.56ava_OperationAndMaintenanceCharges
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Maximum [Member] | Power Option Agreement [Member] | Black Scholes Merton [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
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Average [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Operation and Maintenance Charges 42.90ava_OperationAndMaintenanceCharges
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Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative Liability (25,903)us-gaap_DerivativeLiabilities
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Fair Value, Measurements, Recurring [Member] | Power Option Agreement [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative Liability (251)us-gaap_DerivativeLiabilities
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Fair Value, Measurements, Recurring [Member] | Natural Gas Exchange Agreements [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
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WASHINGTON | Average [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Operation and Maintenance Charges 43.11ava_OperationAndMaintenanceCharges
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IDAHO | Average [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Operation and Maintenance Charges 42.90ava_OperationAndMaintenanceCharges
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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]    
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Fair Value Inputs, Assets, Quantitative Information [Line Items]    
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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, Forward Price 2.10us-gaap_DerivativeForwardPrice
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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, Forward Price 2.34us-gaap_DerivativeForwardPrice
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[1] The estimated volatility rate of 0.20 is compared to actual quoted volatility rates of 0.38 for 2015 to 0.20 in December 2018.
[2] 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.