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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 |  | $ 3,527 us-gaap_CashEquivalentsAtCarryingValue
Equity securities | 36,432 us-gaap_MarketableSecuritiesEquitySecurities | 32,722 us-gaap_MarketableSecuritiesEquitySecurities
Fixed income securities | 32,545 fisi_FixedIncomeSecurities | 28,354 fisi_FixedIncomeSecurities
Total Plan investments | 75,584 us-gaap_InvestmentsAndCash | 64,603 us-gaap_InvestmentsAndCash
Foreign Exchange Contract [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Cash equivalents | 31 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ForeignExchangeContractMember | 92 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ForeignExchangeContractMember
U S Treasury And Government [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Cash equivalents | 249 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember | 937 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember
Fixed income securities | 5,139 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember | 8,984 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember
Municipal Bonds [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fixed income securities |  | 212 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_MunicipalBondsMember
Short Term Investments [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Cash equivalents | 6,327 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ShortTermInvestmentsMember | 2,498 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ShortTermInvestmentsMember
Common Stock [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Equity securities | 14,732 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CommonStockMember | 32,072 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CommonStockMember
Depository Receipts [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Equity securities | 185 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_DepositoryReceiptsMember | 405 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_DepositoryReceiptsMember
Multiemployer Plans Pension [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Equity securities | 10,802 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_MultiemployerPlansPensionMember |
Fixed income securities | 21,083 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_MultiemployerPlansPensionMember |
Exchange Traded Funds [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Equity securities | 10,573 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ExchangeTradedFundsMember |
Preferred Equity [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Equity securities | 140 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PreferredStockMember | 151 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PreferredStockMember
Real Estate Investment [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Equity securities |  | 94 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateInvestmentMember
Consumer Loans Auto Financing Receivable [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fixed income securities | 330 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ConsumerLoansAutoFinancingReceivableMember | 232 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ConsumerLoansAutoFinancingReceivableMember
Collateralized Mortgage Obligations [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fixed income securities | 682 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CollateralizedMortgageObligationsMember | 7,079 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CollateralizedMortgageObligationsMember
All Other Corporate Bonds [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fixed income securities | 2,971 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_AllOtherCorporateBondsMember | 7,609 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_AllOtherCorporateBondsMember
Federal Home Loan Mortgage Corporation [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fixed income securities | 73 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember | 883 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
Federal National Mortgage Association [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fixed income securities | 1,951 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember | 3,044 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Government National Mortgage Association [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fixed income securities |  | 215 fisi_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 | 123 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_GovernmentNationalMortgageAssociationCertificatesAndObligationsGnmaTwoMember | 96 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_GovernmentNationalMortgageAssociationCertificatesAndObligationsGnmaTwoMember
Other Assets [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fixed income securities | 160 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_OtherAssetsMember |
Fixed Income Securities [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fixed income securities | 33 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeSecuritiesMember |
Level 1 Inputs [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Cash equivalents | 31 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 92 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Equity securities | 25,630 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 32,626 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Total Plan investments | 25,661 us-gaap_InvestmentsAndCash / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 32,718 us-gaap_InvestmentsAndCash / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Level 1 Inputs [Member] | Foreign Exchange Contract [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Cash equivalents | 31 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ForeignExchangeContractMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 92 us-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 | 14,732 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CommonStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 32,072 us-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 | 185 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_DepositoryReceiptsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 309 us-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 | 10,573 us-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 | 140 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PreferredStockMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 151 us-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 |  | 94 us-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 | 6,576 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 3,435 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Equity securities | 10,802 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 96 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fixed income securities | 32,545 fisi_FixedIncomeSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 28,354 fisi_FixedIncomeSecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Total Plan investments | 49,923 us-gaap_InvestmentsAndCash / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 31,885 us-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 | 249 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 937 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Fixed income securities | 5,139 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 8,984 fisi_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 |  | 212 fisi_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 | 6,327 us-gaap_CashEquivalentsAtCarryingValue / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 2,498 us-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 |  | 96 us-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 | 10,802 us-gaap_MarketableSecuritiesEquitySecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_MultiemployerPlansPensionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Fixed income securities | 21,083 fisi_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 | 330 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_ConsumerLoansAutoFinancingReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 232 fisi_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 | 682 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CollateralizedMortgageObligationsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 7,079 fisi_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 | 2,971 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_AllOtherCorporateBondsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 7,609 fisi_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 | 73 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 883 fisi_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 | 1,951 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 3,044 fisi_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 |  | 215 fisi_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 | 123 fisi_FixedIncomeSecurities / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = fisi_GovernmentNationalMortgageAssociationCertificatesAndObligationsGnmaTwoMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 96 fisi_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 | 160 fisi_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 | 33 fisi_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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- Definition Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 1 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Sum of investments and unrestricted cash as of the balance sheet date. + References 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) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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