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Share-Based Compensation Plans (Narrative) (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
item
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of share-based compensation plans 2fisi_NumberOfShareBasedCompensationPlans    
Calculation of common stock under incentive plans, each restricted common stock share grant represented amount in shares 1.64fisi_CalculationOfCommonStockUnderIncentivePlansEachCommonStockRepresentedAmountInShares    
Percentage available for issuance as restricted stock grants 61.00%fisi_PercentageAvailableForIssuanceAsRestrictedStockGrants    
Expiration period 10 years    
Unrecognized compensation expense $ 0us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized    
Aggregate intrinsic value 161us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue 106us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue 10us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Total cash received as a result of option exercises 667us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions 448us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions 69us-gaap_EmployeeServiceShareBasedCompensationCashReceivedFromExerciseOfStockOptions
EPS Performance Requirement [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
EPS percentage of award, one year performance period 50.00%fisi_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPercentageCurrentFiscalYearEnd
/ us-gaap_StatementScenarioAxis
= fisi_EarningsPerSharePerformanceRequirementMember
   
TSR Performance Requirement [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Grant date fair value $ 10.54us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_StatementScenarioAxis
= fisi_RelativeTotalShareholderReturnPerformanceRequirementMember
   
TSR percentage of award, three year performance period 50.00%fisi_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPercentageThreeYearPerformancePeriod
/ us-gaap_StatementScenarioAxis
= fisi_RelativeTotalShareholderReturnPerformanceRequirementMember
   
Expected term 2 years 10 months 13 days    
Risk free interest rate 0.62%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_StatementScenarioAxis
= fisi_RelativeTotalShareholderReturnPerformanceRequirementMember
   
Expected dividend yield 3.59%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate
/ us-gaap_StatementScenarioAxis
= fisi_RelativeTotalShareholderReturnPerformanceRequirementMember
   
Expected stock price volatility 39.40%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_StatementScenarioAxis
= fisi_RelativeTotalShareholderReturnPerformanceRequirementMember
   
Restricted Stock Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted shares of common stock awarded 43,242us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Grant date fair value $ 18.76us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Vesting percentage, date of grant 50.00%fisi_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPercentageDateOfGrant
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Vesting percentage, one year from date of grant 50.00%fisi_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPercentageOneYearFromDateOfGrant
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Unrecognized compensation expense $ 488us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Expected recognition expense period, weighted average period in years 1 year 9 months 18 days    
Restricted Stock Awards [Member] | TSR Performance Requirement [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Management Stock Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted shares of common stock awarded 22,642us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_PlanNameAxis
= fisi_ManagementStockIncentivePlanMember
   
Aggregate shares reserved for issuance 690,000us-gaap_DeferredCompensationArrangementWithIndividualCommonStockReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= fisi_ManagementStockIncentivePlanMember
   
Shares available for grant 396,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_PlanNameAxis
= fisi_ManagementStockIncentivePlanMember
   
Management Stock Incentive Plan [Member] | Restricted Stock Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted shares of common stock awarded 11,600us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= fisi_ManagementStockIncentivePlanMember
   
Grant date fair value $ 21.26us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= fisi_ManagementStockIncentivePlanMember
   
Award vesting period 3 years    
Director Stock Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Grant date fair value $ 22.82us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_PlanNameAxis
= fisi_DirectorStockIncentivePlanMember
   
Aggregate shares reserved for issuance 250,000us-gaap_DeferredCompensationArrangementWithIndividualCommonStockReservedForFutureIssuance
/ us-gaap_PlanNameAxis
= fisi_DirectorStockIncentivePlanMember
   
Shares available for grant 119,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_PlanNameAxis
= fisi_DirectorStockIncentivePlanMember
   
Required service period 1 year    
Director Stock Incentive Plan [Member] | Restricted Stock Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted shares of common stock awarded 9,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= fisi_DirectorStockIncentivePlanMember
   
Director Stock Incentive Plan [Member] | Restricted Stock Awards [Member] | Vested Immediately [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted shares of common stock awarded 4,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= fisi_DirectorStockIncentivePlanMember
/ us-gaap_StatementScenarioAxis
= fisi_VestedImmediatelyMember
   
Director Stock Incentive Plan [Member] | Restricted Stock Awards [Member] | Vested After Completion of One-Year Service Requirement [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted shares of common stock awarded 4,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_PlanNameAxis
= fisi_DirectorStockIncentivePlanMember
/ us-gaap_StatementScenarioAxis
= fisi_VestedAfterCompletionOfOneYearServiceRequirementMember
   
Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Minimum [Member] | Restricted Stock Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 2 years    
Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 5 years    
Maximum [Member] | Restricted Stock Awards [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Maximum [Member] | Management Stock Incentive Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Maximum number of grants authorized 500,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= fisi_ManagementStockIncentivePlanMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember