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Investment Securities (Investments' Gross Unrealized Losses And Fair Value) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Investment [Line Items]    
Securities available for sale, Less than 12 months, Fair Value $ 53,910us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue $ 51,852us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Securities available for sale, Less than 12 months, Unrealized Losses 192fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments 116fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
Securities available for sale, 12 months or longer, Fair Value 149,087us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 207,942us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Securities available for sale, 12 months or longer, Unrealized Losses 2,673fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments 5,699fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
Securities available for sale, Fair Value, Total 202,997us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue 259,794us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Securities available for sale, Unrealized Losses, Total 2,865fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments 5,815fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
Securities held to maturity, Less than 12 months, Fair Value 7,670us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue  
Securities held to maturity, Less than 12 months, Unrealized Losses 46fisi_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss  
Securities held to maturity, Fair Value, Total 7,670us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue  
Securities held to maturity, Unrealized Losses, Total 46fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss 120fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Total Securities, Less than 12 months, Fair Value 61,580fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue 69,888fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Total Securities, Less than 12 months, Unrealized Losses 238fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss 236fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Total Securities, 12 months or longer, Fair Value 149,087fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue 207,942fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Total Securities, 12 months or longer, Unrealized Losses 2,673fisi_InvestmentSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss 5,699fisi_InvestmentSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Total Securities, Fair Value 210,667fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionFairValue 277,830fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionFairValue
Total Securities, Unrealized Losses 2,911fisi_InvestmentSecuritiesAccumulatedUnrecognizedHoldingLoss 5,935fisi_InvestmentSecuritiesAccumulatedUnrecognizedHoldingLoss
U.S. Government Agencies And Government Sponsored Enterprises [Member]    
Investment [Line Items]    
Securities available for sale, Less than 12 months, Fair Value 15,444us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
34,995us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, Less than 12 months, Unrealized Losses 42fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
77fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, 12 months or longer, Fair Value 25,092us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
41,070us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, 12 months or longer, Unrealized Losses 262fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
898fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, Fair Value, Total 40,536us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
76,065us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Securities available for sale, Unrealized Losses, Total 304fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
975fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
State And Political Subdivisions [Member]    
Investment [Line Items]    
Securities held to maturity, Less than 12 months, Fair Value 5,484us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
18,036us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities held to maturity, Less than 12 months, Unrealized Losses 28fisi_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
120fisi_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities held to maturity, Fair Value, Total 5,484us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
18,036us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities held to maturity, Unrealized Losses, Total 28fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
120fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Mortgage-Backed Securities [Member]    
Investment [Line Items]    
Securities available for sale, Less than 12 months, Fair Value 38,466us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
16,857us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Less than 12 months, Unrealized Losses 150fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
39fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, 12 months or longer, Fair Value 123,995us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
166,872us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, 12 months or longer, Unrealized Losses 2,411fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
4,801fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value, Total 162,461us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
183,729us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses, Total 2,561fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
4,840fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities held to maturity, Unrealized Losses, Total 18fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
 
Mortgage-Backed Securities [Member] | Federal National Mortgage Association [Member]    
Investment [Line Items]    
Securities available for sale, Less than 12 months, Fair Value 23,349us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
2,242us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Less than 12 months, Unrealized Losses 107fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
8fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, 12 months or longer, Fair Value 34,822us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
62,592us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, 12 months or longer, Unrealized Losses 330fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
1,256fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value, Total 58,171us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
64,834us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses, Total 437fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
1,264fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ 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, Less than 12 months, Fair Value 1,710us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
3,387us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Less than 12 months, Unrealized Losses 8fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
7fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Fair Value, Total 1,710us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
3,387us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
Securities available for sale, Unrealized Losses, Total 8fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
7fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ 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 held to maturity, Less than 12 months, Fair Value 2,186us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
 
Securities held to maturity, Less than 12 months, Unrealized Losses 18fisi_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
 
Securities held to maturity, Fair Value, Total 2,186us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
 
Securities held to maturity, Unrealized Losses, Total 18fisi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember
 
Collateralized Mortgage Obligations [Member]    
Investment [Line Items]    
Securities available for sale, Less than 12 months, Fair Value 13,407us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
11,228us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Less than 12 months, Unrealized Losses 35fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
24fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, 12 months or longer, Fair Value 89,173us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
104,280us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, 12 months or longer, Unrealized Losses 2,081fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
3,545fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value, Total 102,580us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
115,508us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses, Total 2,116fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
3,569fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Collateralized Mortgage Obligations [Member] | Federal National Mortgage Association [Member]    
Investment [Line Items]    
Securities available for sale, Less than 12 months, Fair Value 4,872us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
11,228us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Less than 12 months, Unrealized Losses 22fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
24fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, 12 months or longer, Fair Value 21,119us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
25,644us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, 12 months or longer, Unrealized Losses 561fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
920fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value, Total 25,991us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
36,872us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses, Total 583fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
944fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ 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, Less than 12 months, Fair Value 8,535us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
 
Securities available for sale, Less than 12 months, Unrealized Losses 13fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
 
Securities available for sale, 12 months or longer, Fair Value 65,623us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
76,126us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, 12 months or longer, Unrealized Losses 1,508fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
2,585fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value, Total 74,158us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
76,126us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses, Total 1,521fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
2,585fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ 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, 12 months or longer, Fair Value 2,431us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
2,510us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, 12 months or longer, Unrealized Losses 12fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
40fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Fair Value, Total 2,431us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
2,510us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities available for sale, Unrealized Losses, Total $ 12fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
$ 40fisi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember