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Investment Securities (Summary Of Amortized Cost And Fair Value Of Investment Securities) (Details) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Investment [Line Items] |  |
Securities available for sale, Amortized Cost | $ 630,740us-gaap_AvailableForSaleSecuritiesAmortizedCost | $ 619,293us-gaap_AvailableForSaleSecuritiesAmortizedCost
Securities available for sale, Unrealized Gains | 11,400fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments | 9,016fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
Securities available for sale, Unrealized Losses | 2,865fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments | 5,815fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Securities available for sale, Fair Value | 639,275us-gaap_AvailableForSaleSecurities | 622,494us-gaap_AvailableForSaleSecurities
Securities held to maturity, Amortized Cost | 306,255us-gaap_HeldToMaturitySecurities | 294,438us-gaap_HeldToMaturitySecurities
Securities held to maturity, Unrealized Gains | 6,301fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain | 4,377fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Securities held to maturity, Unrealized Losses | 46fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss | 120fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Securities held to maturity, Fair Value | 312,510us-gaap_HeldToMaturitySecuritiesFairValue | 298,695us-gaap_HeldToMaturitySecuritiesFairValue
U.S. Government Agencies And Government Sponsored Enterprises [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Amortized Cost | 176,648us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 160,334us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, Unrealized Gains | 2,324fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 1,116fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, Unrealized Losses | 304fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 975fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, Fair Value | 178,668us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 160,475us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
State And Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Securities held to maturity, Amortized Cost | 285,630us-gaap_HeldToMaturitySecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 277,273us-gaap_HeldToMaturitySecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities held to maturity, Unrealized Gains | 6,121fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 4,231fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities held to maturity, Unrealized Losses | 28fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 120fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities held to maturity, Fair Value | 291,723us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 281,384us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Amortized Cost | 454,092us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 458,959us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Gains | 8,845fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 7,669fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses | 2,561fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 4,840fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value | 460,376us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 461,788us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Amortized Cost | 20,625us-gaap_HeldToMaturitySecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 17,165us-gaap_HeldToMaturitySecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Unrealized Gains | 180fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 146fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Unrealized Losses | 18fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Securities held to maturity, Fair Value | 20,787us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 17,311us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Mortgage-Backed Securities [Member] | Federal National Mortgage Association [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Amortized Cost | 187,368us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 184,857us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Gains | 3,067fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 2,344fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses | 437fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 1,264fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value | 189,998us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 185,937us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Amortized Cost | 3,253us-gaap_HeldToMaturitySecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 3,279us-gaap_HeldToMaturitySecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Unrealized Gains | 55fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 24fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Fair Value | 3,308us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 3,303us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Mortgage-Backed Securities [Member] | Federal Home Loan Mortgage Corporation [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Amortized Cost | 25,892us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 29,478us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Gains | 769fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 799fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses | 8fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 7fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value | 26,653us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 30,270us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Mortgage-Backed Securities [Member] | Government National Mortgage Association [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Amortized Cost | 46,584us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 48,800us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Gains | 2,093fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 2,022fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value | 48,677us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 50,822us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Amortized Cost | 17,372us-gaap_HeldToMaturitySecurities / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 13,886us-gaap_HeldToMaturitySecurities / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Unrealized Gains | 125fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 122fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Unrealized Losses | 18fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Securities held to maturity, Fair Value | 17,479us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 14,008us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Collateralized Mortgage Obligations [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Amortized Cost | 194,248us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 195,824us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Gains | 2,916fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 2,504fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses | 2,116fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 3,569fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value | 195,048us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 194,759us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Collateralized Mortgage Obligations [Member] | Federal National Mortgage Association [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Amortized Cost | 76,408us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 76,247us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Gains | 969fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 489fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses | 583fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 944fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value | 76,794us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 75,792us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Collateralized Mortgage Obligations [Member] | Federal Home Loan Mortgage Corporation [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Amortized Cost | 90,875us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 89,623us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Gains | 233fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 199fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses | 1,521fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 2,585fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value | 89,587us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 87,237us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Collateralized Mortgage Obligations [Member] | Government National Mortgage Association [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Amortized Cost | 26,965us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 29,954us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Gains | 545fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 598fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses | 12fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 40fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value | 27,498us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 30,512us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Collateralized Mortgage Obligations [Member] | Privately Issued [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Unrealized Gains | 1,169fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 1,218fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value | 1,169us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 1,218us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Unrealized Gains | 231fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 231fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Securities available for sale, Fair Value | $ 231us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | $ 231us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember

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- Definition Amount of accumulated pre-tax unrealized gains before deducting pre-tax unrealized losses on investments in available-for-sale securities impacting investments. + References No definition available. + Details
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