XML 20 R55.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
Dec. 31, 2014
Dec. 31, 2013
Investment [Line Items]    
Securities available for sale, Amortized Cost $ 619,293us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 618,166us-gaap_AvailableForSaleSecuritiesAmortizedCost
Securities available for sale, Unrealized Gains 9,016fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments 8,830fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
Securities available for sale, Unrealized Losses 5,815fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments 17,596fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Securities available for sale, Fair Value 622,494us-gaap_AvailableForSaleSecurities 609,400us-gaap_AvailableForSaleSecurities
Securities held to maturity, Amortized Cost 294,438us-gaap_HeldToMaturitySecurities 249,785us-gaap_HeldToMaturitySecurities
Securities held to maturity, Unrealized Gains 4,377fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain  
Securities held to maturity, Unrealized Losses 120fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss  
Securities held to maturity, Fair Value 298,695us-gaap_HeldToMaturitySecuritiesFairValue 250,657us-gaap_HeldToMaturitySecuritiesFairValue
U.S. Government Agencies And Government Sponsored Enterprises [Member]    
Investment [Line Items]    
Securities available for sale, Amortized Cost 160,334us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
135,840us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, Unrealized Gains 1,116fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
1,414fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, Unrealized Losses 975fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
2,802fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, Fair Value 160,475us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
134,452us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
State And Political Subdivisions [Member]    
Investment [Line Items]    
Securities held to maturity, Amortized Cost 277,273us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
249,785us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities held to maturity, Unrealized Gains 4,231fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
1,340fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities held to maturity, Unrealized Losses 120fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
468fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities held to maturity, Fair Value 281,384us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
250,657us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Securities available for sale, Amortized Cost 458,959us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
482,308us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Gains 7,669fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
7,035fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses 4,840fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
14,794fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value 461,788us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
474,549us-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 184,857us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
173,507us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Gains 2,344fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
1,511fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses 1,264fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
4,810fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value 185,937us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
170,208us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Amortized Cost 3,279us-gaap_HeldToMaturitySecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
 
Securities held to maturity, Unrealized Gains 24fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
 
Securities held to maturity, Fair Value 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 29,478us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
36,737us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Gains 799fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
562fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses 7fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
205fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value 30,270us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
37,094us-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 48,800us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
61,832us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Gains 2,022fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
2,152fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses   142fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value 50,822us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
63,842us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Amortized Cost 13,886us-gaap_HeldToMaturitySecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
 
Securities held to maturity, Unrealized Gains 122fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
 
Securities held to maturity, Fair Value 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 195,824us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
210,232us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Gains 2,504fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
2,810fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses 3,569fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
9,637fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value 194,759us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
203,405us-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 76,247us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
63,838us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Gains 489fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
261fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses 944fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
3,195fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value 75,792us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
60,904us-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 89,623us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
102,660us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Gains 199fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
169fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses 2,585fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
5,856fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value 87,237us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
96,973us-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 29,954us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
43,734us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Gains 598fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
913fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses 40fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
586fisi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value 30,512us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
44,061us-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 1,218fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
1,467fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value 1,218us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
1,467us-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   18us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Securities available for sale, Unrealized Gains 231fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
381fisi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Securities available for sale, Fair Value $ 231us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
$ 399us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember