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Discontinued Operations Summary of Cash Proceeds from Sale of Discontinued Operations (Details) (USD $)
3 Months Ended 12 Months Ended
Mar. 31, 2015
M
Mar. 31, 2014
Dec. 31, 2014
Jun. 30, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Contract price       $ 335,000,000us-gaap_DisposalGroupIncludingDiscontinuedOperationConsideration
Repayment of long-term borrowings under committed line of credit 0us-gaap_RepaymentsOfLinesOfCredit (4,000,000)us-gaap_RepaymentsOfLinesOfCredit    
Transaction expenses withheld from proceeds     (5,390,000)ava_TransactionExpensesWithheldfromSalesProceeds  
Total net proceeds related to sales transaction     143,500,000us-gaap_ProceedsFromDivestitureOfBusinesses  
Number of Months Receivable Held in Escrow 15ava_NumberofMonthsReceivableHeldinEscrow      
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax     69,700,000us-gaap_DiscontinuedOperationGainLossOnDisposalOfDiscontinuedOperationNetOfTax  
Indemnification Agreement [Member]        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Escrow Receivable 16,800,000ava_EscrowReceivable
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_IndemnificationGuaranteeMember
     
Escrow Percentage of Contract Price 5.00%ava_EscrowPercentageofContractPrice
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_IndemnificationGuaranteeMember
     
Working Capital Escrow Adjustment [Member]        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Escrow Receivable $ 1,000,000ava_EscrowReceivable
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= ava_WorkingCapitalEscrowAdjustmentMember