XML 16 R54.htm IDEA: XBRL DOCUMENT v2.4.1.9
Investment Securities (Narrative) (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
security
Dec. 31, 2012
Dec. 31, 2013
security
Investment Securities [Abstract]      
Securities pledged as collateral $ 768,600,000us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral   $ 763,100,000us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
Securities transferred from available for sale to held to maturity (at fair value) 12,800,000fisi_SecuritiesTransferredFromAvailableForSaleToHeldToMaturityAtFairValue    
Unrealized holding losses included in accumulated other comprehensive income and in the carrying value of the held to maturity securities 51,000fisi_OtherComprehensiveIncomeLossTransfersFromAvailableForSaleToHeldToMaturitySecuritiesBeforeTax    
Number of security positions, unrealized loss position 122us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1   331us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Number of security positions, unrealized loss position for more than 12 months 51fisi_AvailableForSaleSecuritiesInUnrealizedLossPositionsTwelveMonthsOrLongerQualitativeDisclosureNumberOfPositions   14fisi_AvailableForSaleSecuritiesInUnrealizedLossPositionsTwelveMonthsOrLongerQualitativeDisclosureNumberOfPositions
Fair value, continuous unrealized loss position for more than 12 months 207,942,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue   50,928,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Securities available for sale, 12 months or longer, Unrealized Losses 5,699,000fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments   2,818,000fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
Number of security positions, unrealized loss position for less than 12 months 71fisi_AvailableForSaleSecuritiesInUnrealizedLossPositionsLessThanTwelveMonthsQualitativeDisclosureNumberOfPositions   317fisi_AvailableForSaleSecuritiesInUnrealizedLossPositionsLessThanTwelveMonthsQualitativeDisclosureNumberOfPositions
Securities, less than 12 months, Fair Value 69,888,000fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue   407,299,000fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Securities, less than 12 months, Unrealized Losses 236,000fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss   15,246,000fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
OTTI charge recognized   $ 91,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet