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Fair Value (Quantitative Information) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Liability (229,790)us-gaap_DerivativeFairValueOfDerivativeLiability $ (89,675)us-gaap_DerivativeFairValueOfDerivativeLiability
Operation and Maintenance Charges 42.90ava_OperationAndMaintenanceCharges  
Power Exchange Agreements [Member] | 2016 to 2019 [Member] | Surrogate Facility Pricing [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Escalation Factor 3.00%ava_EscalationFactor
/ us-gaap_DerivativeByNatureAxis
= ava_TwoThousandFifteenToTwoThousandNineteenMember
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerExchangeAgreementsMember
/ us-gaap_ValuationTechniqueAxis
= ava_SurrogateFacilityPricingMember		  
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
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerOptionAgreementMember
/ us-gaap_ValuationTechniqueAxis
= ava_BlackScholesMertonMember		 [1]  
Power Option Agreement [Member] | 2015 | Black Scholes Merton [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Price Risk Option Strike Price 41.20us-gaap_DerivativePriceRiskOptionStrikePrice
/ us-gaap_DerivativeByNatureAxis
= ava_TwoThousandFifteenMember
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerOptionAgreementMember
/ us-gaap_ValuationTechniqueAxis
= ava_BlackScholesMertonMember		  
Fair Value Assumptions, Expected Volatility Rate 45.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate
/ us-gaap_DerivativeByNatureAxis
= ava_TwoThousandFifteenMember
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerOptionAgreementMember
/ us-gaap_ValuationTechniqueAxis
= ava_BlackScholesMertonMember		  
Power Option Agreement [Member] | 2017 | Black Scholes Merton [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair Value Assumptions, Expected Volatility Rate 21.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate
/ us-gaap_DerivativeByNatureAxis
= ava_TwoThousandSeventeenMember
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerOptionAgreementMember
/ us-gaap_ValuationTechniqueAxis
= ava_BlackScholesMertonMember		  
Power Option Agreement [Member] | 2019 [Member] | Black Scholes Merton [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Price Risk Option Strike Price 64.09us-gaap_DerivativePriceRiskOptionStrikePrice
/ us-gaap_DerivativeByNatureAxis
= ava_TwoThousandNineteenMember
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerOptionAgreementMember
/ us-gaap_ValuationTechniqueAxis
= ava_BlackScholesMertonMember		  
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
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerExchangeAgreementsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= ava_SurrogateFacilityPricingMember		 [2]  
Derivative, Nonmonetary Notional Amount, Energy Measure 184,077us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerExchangeAgreementsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= ava_SurrogateFacilityPricingMember		  
Minimum [Member] | Power Option Agreement [Member] | Black Scholes Merton [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Nonmonetary Notional Amount, Energy Measure 128,491us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerOptionAgreementMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= ava_BlackScholesMertonMember		  
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
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerExchangeAgreementsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= ava_SurrogateFacilityPricingMember		 [2]  
Derivative, Nonmonetary Notional Amount, Energy Measure 397,116us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerExchangeAgreementsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= ava_SurrogateFacilityPricingMember		  
Maximum [Member] | Power Option Agreement [Member] | Black Scholes Merton [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Nonmonetary Notional Amount, Energy Measure 287,147us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerOptionAgreementMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= ava_BlackScholesMertonMember		  
Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Liability (23,299)us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerExchangeAgreementsMember
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember		  
Fair Value, Measurements, Recurring [Member] | Power Option Agreement [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Liability (424)us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_PowerOptionAgreementMember
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember		  
Fair Value, Measurements, Recurring [Member] | Natural Gas Exchange Agreements [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Liability (35)us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_NaturalGasExchangeAgreementsMember
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsRecurringMember		  
WASHINGTON    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Operation and Maintenance Charges 43.11ava_OperationAndMaintenanceCharges
/ us-gaap_StatutoryAccountingPracticesByJurisdictionAxis
= stpr_WA		  
IDAHO    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Operation and Maintenance Charges 42.90ava_OperationAndMaintenanceCharges
/ us-gaap_StatutoryAccountingPracticesByJurisdictionAxis
= stpr_ID		  
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 279,990us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_NaturalGasExchangeAgreementsMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= ava_InternallyDerivedWeightedAverageCostOfGasMember		  
Derivative, Forward Price 2.56us-gaap_DerivativeForwardPrice
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_NaturalGasExchangeAgreementsMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= ava_InternallyDerivedWeightedAverageCostOfGasMember		  
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 365,118us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_NaturalGasExchangeAgreementsMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= ava_InternallyDerivedWeightedAverageCostOfGasMember		  
Derivative, Forward Price 3.53us-gaap_DerivativeForwardPrice
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_NaturalGasExchangeAgreementsMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= ava_InternallyDerivedWeightedAverageCostOfGasMember		  
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 280,000us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_NaturalGasExchangeAgreementsMember
/ us-gaap_IncomeStatementLocationAxis
= ava_PurchaseMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= ava_InternallyDerivedWeightedAverageCostOfGasMember		  
Derivative, Forward Price 2.32us-gaap_DerivativeForwardPrice
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_NaturalGasExchangeAgreementsMember
/ us-gaap_IncomeStatementLocationAxis
= ava_PurchaseMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ValuationTechniqueAxis
= ava_InternallyDerivedWeightedAverageCostOfGasMember		  
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 310,000us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_NaturalGasExchangeAgreementsMember
/ us-gaap_IncomeStatementLocationAxis
= ava_PurchaseMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= ava_InternallyDerivedWeightedAverageCostOfGasMember		  
Derivative, Forward Price 2.57us-gaap_DerivativeForwardPrice
/ us-gaap_DerivativeInstrumentRiskAxis
= ava_NaturalGasExchangeAgreementsMember
/ us-gaap_IncomeStatementLocationAxis
= ava_PurchaseMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ValuationTechniqueAxis
= ava_InternallyDerivedWeightedAverageCostOfGasMember		  
[1] The estimated volatility rate of 0.20 is compared to actual quoted volatility rates of 0.45 for 2015 to 0.21 in December 2017.
[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.