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Long-Term Debt To Affiliated Trusts (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2000
Dec. 31, 2014
Dec. 31, 1997
Junior Subordinated Deferrable Interest Debentures series B, principal amount   51.5us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust $ 51.5us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
Purchase of preferred trust securities 10.0us-gaap_PaymentsForRepurchaseOfTrustPreferredSecurities    
Ownership interest   100.00%us-gaap_EquityMethodInvestmentOwnershipPercentage  
Trust Preferred Securities Subject to Mandatory Redemption [Member]      
Issuance of trust securities     50.0us-gaap_ProceedsFromIssuanceOfTrustPreferredSecurities
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
Description of variable rate basis   LIBOR  
Basis spread on variable rate   0.875%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
 
Common Trust Securities [Member]      
Issuance of trust securities   1.5us-gaap_ProceedsFromIssuanceOfTrustPreferredSecurities
/ us-gaap_LongtermDebtTypeAxis
= ava_CommonTrustSecuritiesMember