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Investment Securities (Summary Of Amortized Cost And Fair Value Of Investment Securities) (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Investment [Line Items] |  |
Securities available for sale, Amortized Cost | $ 619,293 us-gaap_AvailableForSaleSecuritiesAmortizedCost | $ 618,166 us-gaap_AvailableForSaleSecuritiesAmortizedCost
Securities available for sale, Unrealized Gains | 9,016 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments | 8,830 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
Securities available for sale, Unrealized Losses | 5,815 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments | 17,596 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Securities available for sale, Fair Value | 622,494 us-gaap_AvailableForSaleSecurities | 609,400 us-gaap_AvailableForSaleSecurities
Securities held to maturity, Amortized Cost | 294,438 us-gaap_HeldToMaturitySecurities | 249,785 us-gaap_HeldToMaturitySecurities
Securities held to maturity, Unrealized Gains | 4,377 fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain |
Securities held to maturity, Unrealized Losses | 120 fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss |
Securities held to maturity, Fair Value | 298,695 us-gaap_HeldToMaturitySecuritiesFairValue | 250,657 us-gaap_HeldToMaturitySecuritiesFairValue
U.S. Government Agencies And Government Sponsored Enterprises [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Amortized Cost | 160,334 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 135,840 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, Unrealized Gains | 1,116 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 1,414 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, Unrealized Losses | 975 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 2,802 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, Fair Value | 160,475 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 134,452 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
State And Political Subdivisions [Member] |  |
Investment [Line Items] |  |
Securities held to maturity, Amortized Cost | 277,273 us-gaap_HeldToMaturitySecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 249,785 us-gaap_HeldToMaturitySecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities held to maturity, Unrealized Gains | 4,231 fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 1,340 fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities held to maturity, Unrealized Losses | 120 fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 468 fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities held to maturity, Fair Value | 281,384 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 250,657 us-gaap_HeldToMaturitySecuritiesFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Mortgage-Backed Securities [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Amortized Cost | 458,959 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 482,308 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Gains | 7,669 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 7,035 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses | 4,840 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 14,794 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value | 461,788 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 474,549 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Mortgage-Backed Securities [Member] | Federal National Mortgage Association [Member] |  |
Investment [Line Items] |  |
Securities available for sale, Amortized Cost | 184,857 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 173,507 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Gains | 2,344 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 1,511 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses | 1,264 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 4,810 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value | 185,937 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 170,208 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Amortized Cost | 3,279 us-gaap_HeldToMaturitySecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Securities held to maturity, Unrealized Gains | 24 fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Securities held to maturity, Fair Value | 3,303 us-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 | 29,478 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 36,737 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Gains | 799 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 562 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses | 7 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 205 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value | 30,270 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 37,094 us-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 | 48,800 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 61,832 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Gains | 2,022 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 2,152 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses |  | 142 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value | 50,822 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember | 63,842 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Amortized Cost | 13,886 us-gaap_HeldToMaturitySecurities / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Securities held to maturity, Unrealized Gains | 122 fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Securities held to maturity, Fair Value | 14,008 us-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 | 195,824 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 210,232 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Gains | 2,504 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 2,810 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses | 3,569 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 9,637 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value | 194,759 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 203,405 us-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 | 76,247 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 63,838 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Gains | 489 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 261 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses | 944 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 3,195 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value | 75,792 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 60,904 us-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 | 89,623 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 102,660 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Gains | 199 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 169 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses | 2,585 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 5,856 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value | 87,237 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 96,973 us-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 | 29,954 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 43,734 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Gains | 598 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 913 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses | 40 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 586 fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value | 30,512 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 44,061 us-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 | 1,218 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 1,467 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_InvestmentTypeAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value | 1,218 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 1,467 us-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, Amortized Cost |  | 18 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Securities available for sale, Unrealized Gains | 231 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 381 fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Securities available for sale, Fair Value | $ 231 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | $ 399 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember

X
- 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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- Definition Amount of accumulated pre-tax unrealized loss before deducting pre-tax unrealized gain on investments in available-for-sale securities impacting investments. + References No definition available. + Details
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- Definition This item represents the excess of amortized cost basis over fair value of securities in a gain position and which are categorized as held-to-maturity. + References No definition available. + Details
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- Definition This item represents the excess of amortized cost basis over fair value of securities in a loss position and which are categorized as held-to-maturity. + References No definition available. + Details
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- Details
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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.6) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Fair value of investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 + Details
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