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Common Stock Dividends Declared (Details) (USD $)
In Millions, except Per Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dividend Restrictions [Line Items]      
Maximum Dividends Allowed by Debt Covenants $ 295.0ava_MaximumDividendsAllowedbyDebtCovenants    
Maximum Dividends Allowed by Regulator Approval $ 145.0ava_MaximumDividendsAllowedbyRegulatorApproval    
Dividends declared per common share $ 1.27us-gaap_CommonStockDividendsPerShareCashPaid $ 1.22us-gaap_CommonStockDividendsPerShareCashPaid $ 1.16us-gaap_CommonStockDividendsPerShareCashPaid
Maximum [Member]      
Dividend Restrictions [Line Items]      
Ratio of Equity to Total Capitalization 40.00%ava_RatioofEquitytoTotalCapitalization
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		    
Avista Corporation [Member]      
Dividend Restrictions [Line Items]      
Line of Credit Facility, Covenant Terms 0.65