XML 82 R41.htm IDEA: XBRL DOCUMENT v2.4.1.9
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
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