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Employee Benefit Plans (Narrative) (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation $ 54,900,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation $ 44,000,000us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation  
Defined Benefit Plan, Contributions by Employer 8,000,000us-gaap_DefinedBenefitPlanContributionsByEmployer    
Number of consecutive years used for compensation calculation 5 years    
Investment strategy, percentage in long-term growth 97.00%fisi_InvestmentStrategyPercentageInLongTermGrowth    
Investment strategy, percentage in near-term benefit payments 3.00%fisi_InvestmentStrategyPercentageInNearTermBenefitPayments    
Maximum percentage of securities purchasing allowed of the portfolio at any time 5.00%fisi_MaximumPercentageOfSecuritiesPurchasingAllowedOfPortfolioAtAnyTime    
Maximum market value purchase percentage allowed of any one issuer 8.00%fisi_MaximumMarketValuePurchasePercentageAllowedOfAnyOneIssuer    
Expense included in salaries and employee benefits 1,100,000us-gaap_DefinedContributionPlanCostRecognized 1,100,000us-gaap_DefinedContributionPlanCostRecognized 1,000,000us-gaap_DefinedContributionPlanCostRecognized
Postretirement Benefit Plan, Accrued liabilities 124,000us-gaap_PensionAndOtherPostretirementAndPostemploymentBenefitPlansLiabilitiesCurrentAndNoncurrent 93,000us-gaap_PensionAndOtherPostretirementAndPostemploymentBenefitPlansLiabilitiesCurrentAndNoncurrent  
Estimated future net loss for the plan that will be amortized from accumulated other comprehensive income 943,000us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss    
Estimated future prior service credit for the plan that will be amortized from accumulated other comprehensive income 47,000us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit    
Maximum percent of participant's contribution match 4.50%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent    
Effective February Two Thousand Twelve [Member] | Securities Less Than Aquality [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Maximum percentage of securities purchasing allowed of the portfolio at any time 13.00%fisi_MaximumPercentageOfSecuritiesPurchasingAllowedOfPortfolioAtAnyTime
/ us-gaap_InternalCreditAssessmentAxis
= fisi_SecuritiesLessThanAqualityMember
/ us-gaap_StatementScenarioAxis
= fisi_EffectiveFebruaryTwoThousandTwelveMember
   
Effective February Two Thousand Twelve [Member] | Commercial Mortgage Back Securities And Assets Backed Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Maximum percentage of securities purchasing allowed of the portfolio at any time 10.00%fisi_MaximumPercentageOfSecuritiesPurchasingAllowedOfPortfolioAtAnyTime
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fisi_CommercialMortgageBackSecuritiesAndAssetsBackedSecuritiesMember
/ us-gaap_StatementScenarioAxis
= fisi_EffectiveFebruaryTwoThousandTwelveMember
   
Street Bank & Trust Co. Short Term Investment Fund [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Percentage of Plan Assets   5.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= fisi_StreetBankTrustCoShortTermInvestmentFundMember
 
Portfolio Concentration Commingled Trust Fund1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Percentage of Plan Assets 13.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= fisi_PortfolioConcentrationCommingledTrustFund1Member
   
Portfolio Concentration Commingled Trust Fund2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Percentage of Plan Assets 9.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= fisi_PortfolioConcentrationCommingledTrustFund2Member
   
Portfolio Concentration Short Term Investment Fund [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Percentage of Plan Assets 8.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= fisi_PortfolioConcentrationShortTermInvestmentFundMember
   
Investment Firm One [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Portfolio management, controlled percentage 57.00%fisi_PortfolioManagementControlledPercentage
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= fisi_InvestmentFirmOneMember
58.00%fisi_PortfolioManagementControlledPercentage
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= fisi_InvestmentFirmOneMember
 
Investment Firm Two [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Portfolio management, controlled percentage 39.00%fisi_PortfolioManagementControlledPercentage
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= fisi_InvestmentFirmTwoMember
41.00%fisi_PortfolioManagementControlledPercentage
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= fisi_InvestmentFirmTwoMember
 
Defined Benefit Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Portfolio management, controlled percentage 4.00%fisi_PortfolioManagementControlledPercentage
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
1.00%fisi_PortfolioManagementControlledPercentage
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
 
First 3% Of Participant's Contribution [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Percentage employer matches of the employee's percentage contribution matched 100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= fisi_FirstThreePercentOfParticipantSContributionMember
   
Percentage for which the employer contributes a matching contribution 3.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= fisi_FirstThreePercentOfParticipantSContributionMember
   
Next 3% Of Participant's Contribution [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Percentage employer matches of the employee's percentage contribution matched 50.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= fisi_NextThreePercentOfParticipantSContributionMember
   
Percentage for which the employer contributes a matching contribution 3.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= fisi_NextThreePercentOfParticipantSContributionMember
   
SERP [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Unfunded pension liability 2,200,000us-gaap_DefinedBenefitPensionPlanCurrentAndNoncurrentLiabilities
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
2,100,000us-gaap_DefinedBenefitPensionPlanCurrentAndNoncurrentLiabilities
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
 
Pension expense $ 295,000us-gaap_PensionExpense
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
$ 95,000us-gaap_PensionExpense
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember
$ 1,300,000us-gaap_PensionExpense
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember