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Investment Securities (Narrative) (Details) (USD $)
 3 Months Ended
Mar. 31, 2015
security
Mar. 31, 2014
Dec. 31, 2014
security
Investment Securities [Abstract]      
Securities pledged as collateral $ 849,300,000us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral    
Number of security positions, unrealized loss position 74us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1   122us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Number of security positions, unrealized loss position for more than 12 months 33fisi_AvailableForSaleSecuritiesInUnrealizedLossPositionsTwelveMonthsOrLongerQualitativeDisclosureNumberOfPositions   51fisi_AvailableForSaleSecuritiesInUnrealizedLossPositionsTwelveMonthsOrLongerQualitativeDisclosureNumberOfPositions
Fair value, continuous unrealized loss position for more than 12 months 149,087,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue   207,942,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Securities available for sale, 12 months or longer, Unrealized Losses 2,673,000fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments   5,699,000fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
Number of security positions, unrealized loss position for less than 12 months 41fisi_AvailableForSaleSecuritiesInUnrealizedLossPositionsLessThanTwelveMonthsQualitativeDisclosureNumberOfPositions   71fisi_AvailableForSaleSecuritiesInUnrealizedLossPositionsLessThanTwelveMonthsQualitativeDisclosureNumberOfPositions
Securities, less than 12 months, Fair Value 61,580,000fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue   69,888,000fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Securities, less than 12 months, Unrealized Losses 238,000fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss   236,000fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
OTTI charge recognized $ 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet $ 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet