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Employee Benefit Plans (The Major Categories Of Plan Assets Measured At Fair Value On a Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Cash equivalents		$ 3,527us-gaap_CashEquivalentsAtCarryingValue
Equity securities	36,432us-gaap_MarketableSecuritiesEquitySecurities	32,722us-gaap_MarketableSecuritiesEquitySecurities
Fixed income securities	32,545fisi_FixedIncomeSecurities	28,354fisi_FixedIncomeSecurities
Total Plan investments	75,584us-gaap_InvestmentsAndCash	64,603us-gaap_InvestmentsAndCash
Foreign Exchange Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash equivalents	31us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ForeignExchangeContractMember	92us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ForeignExchangeContractMember
U S Treasury And Government [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash equivalents	249us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember	937us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember
Fixed income securities	5,139fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember	8,984fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember
Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities		212fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_MunicipalBondsMember
Short Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash equivalents	6,327us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ShortTermInvestmentsMember	2,498us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ShortTermInvestmentsMember
Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities	14,732us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CommonStockMember	32,072us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CommonStockMember
Depository Receipts [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities	185us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_DepositoryReceiptsMember	405us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_DepositoryReceiptsMember
Multiemployer Plans Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities	10,802us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_MultiemployerPlansPensionMember
Fixed income securities	21,083fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_MultiemployerPlansPensionMember
Exchange Traded Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities	10,573us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ExchangeTradedFundsMember
Preferred Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities	140us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PreferredStockMember	151us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PreferredStockMember
Real Estate Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities		94us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateInvestmentMember
Consumer Loans Auto Financing Receivable [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	330fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ConsumerLoansAutoFinancingReceivableMember	232fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ConsumerLoansAutoFinancingReceivableMember
Collateralized Mortgage Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	682fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CollateralizedMortgageObligationsMember	7,079fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CollateralizedMortgageObligationsMember
All Other Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	2,971fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_AllOtherCorporateBondsMember	7,609fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_AllOtherCorporateBondsMember
Federal Home Loan Mortgage Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	73fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember	883fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
Federal National Mortgage Association [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	1,951fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember	3,044fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Government National Mortgage Association [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities		215fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
Government National Mortgage Association Certificates And Obligations Gnma Two [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	123fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_GovernmentNationalMortgageAssociationCertificatesAndObligationsGnmaTwoMember	96fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_GovernmentNationalMortgageAssociationCertificatesAndObligationsGnmaTwoMember
Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	160fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_OtherAssetsMember
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	33fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeSecuritiesMember
Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash equivalents	31us-gaap_CashEquivalentsAtCarryingValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member	92us-gaap_CashEquivalentsAtCarryingValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Equity securities	25,630us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member	32,626us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Total Plan investments	25,661us-gaap_InvestmentsAndCash / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member	32,718us-gaap_InvestmentsAndCash / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Level 1 Inputs [Member] | Foreign Exchange Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash equivalents	31us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ForeignExchangeContractMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member	92us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ForeignExchangeContractMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Level 1 Inputs [Member] | Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities	14,732us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CommonStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member	32,072us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CommonStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Level 1 Inputs [Member] | Depository Receipts [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities	185us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_DepositoryReceiptsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member	309us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_DepositoryReceiptsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Level 1 Inputs [Member] | Exchange Traded Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities	10,573us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ExchangeTradedFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Level 1 Inputs [Member] | Preferred Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities	140us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PreferredStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member	151us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PreferredStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Level 1 Inputs [Member] | Real Estate Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities		94us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateInvestmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash equivalents	6,576us-gaap_CashEquivalentsAtCarryingValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	3,435us-gaap_CashEquivalentsAtCarryingValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Equity securities	10,802us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	96us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fixed income securities	32,545fisi_FixedIncomeSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	28,354fisi_FixedIncomeSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Total Plan investments	49,923us-gaap_InvestmentsAndCash / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	31,885us-gaap_InvestmentsAndCash / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | U S Treasury And Government [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash equivalents	249us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	937us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fixed income securities	5,139fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	8,984fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities		212fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_MunicipalBondsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | Short Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash equivalents	6,327us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	2,498us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | Depository Receipts [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities		96us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_DepositoryReceiptsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | Multiemployer Plans Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities	10,802us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_MultiemployerPlansPensionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fixed income securities	21,083fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_MultiemployerPlansPensionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | Consumer Loans Auto Financing Receivable [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	330fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ConsumerLoansAutoFinancingReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	232fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ConsumerLoansAutoFinancingReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | Collateralized Mortgage Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	682fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CollateralizedMortgageObligationsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	7,079fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CollateralizedMortgageObligationsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | All Other Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	2,971fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_AllOtherCorporateBondsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	7,609fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_AllOtherCorporateBondsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | Federal Home Loan Mortgage Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	73fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	883fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | Federal National Mortgage Association [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	1,951fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	3,044fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | Government National Mortgage Association [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities		215fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | Government National Mortgage Association Certificates And Obligations Gnma Two [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	123fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_GovernmentNationalMortgageAssociationCertificatesAndObligationsGnmaTwoMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member	96fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_GovernmentNationalMortgageAssociationCertificatesAndObligationsGnmaTwoMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	160fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_OtherAssetsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 2 Inputs [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities	33fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 3 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash equivalents
Equity securities
Fixed income securities
Total Plan investments
Level 3 Inputs [Member] | Foreign Exchange Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash equivalents
Level 3 Inputs [Member] | U S Treasury And Government [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash equivalents
Fixed income securities
Level 3 Inputs [Member] | Municipal Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities
Level 3 Inputs [Member] | Short Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash equivalents
Level 3 Inputs [Member] | Common Stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities
Level 3 Inputs [Member] | Depository Receipts [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities
Level 3 Inputs [Member] | Multiemployer Plans Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities
Fixed income securities
Level 3 Inputs [Member] | Exchange Traded Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities
Level 3 Inputs [Member] | Preferred Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities
Level 3 Inputs [Member] | Real Estate Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Equity securities
Level 3 Inputs [Member] | Consumer Loans Auto Financing Receivable [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities
Level 3 Inputs [Member] | Collateralized Mortgage Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities
Level 3 Inputs [Member] | All Other Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities
Level 3 Inputs [Member] | Federal Home Loan Mortgage Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities
Level 3 Inputs [Member] | Federal National Mortgage Association [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities
Level 3 Inputs [Member] | Government National Mortgage Association [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities
Level 3 Inputs [Member] | Government National Mortgage Association Certificates And Obligations Gnma Two [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities
Level 3 Inputs [Member] | Other Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities
Level 3 Inputs [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fixed income securities

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Fixed Income Securities

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

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-Publisher SEC

-Name Regulation S-X (SX)

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-Section 02

-Paragraph 1

-Article 5



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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

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-Section S99

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Sum of investments and unrestricted cash as of the balance sheet date.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.1,2)

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph b

-Article 7



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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

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