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Loans (Commercial Loan Portfolio Categorized By Internally Assigned Asset Classification) (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]    
Total Loans $ 1,881,071us-gaap_LoansAndLeasesReceivableGrossCarryingAmount $ 1,802,105us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Commercial Business [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total Loans 267,377us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fisi_CommercialAndIndustrialMember
265,751us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fisi_CommercialAndIndustrialMember
Commercial Business [Member] | Uncriticized [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total Loans 250,961us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fisi_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
250,553us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fisi_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Commercial Business [Member] | Special Mention [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total Loans 5,530us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fisi_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
6,311us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fisi_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Commercial Business [Member] | Substandard [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total Loans 10,886us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fisi_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
8,887us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fisi_CommercialAndIndustrialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Commercial Business [Member] | Doubtful [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total Loans      
Commercial Mortgage [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total Loans 476,407us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
470,312us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
Commercial Mortgage [Member] | Uncriticized [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total Loans 460,880us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
449,447us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Commercial Mortgage [Member] | Special Mention [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total Loans 5,411us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
6,895us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
Commercial Mortgage [Member] | Substandard [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total Loans 10,116us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
13,970us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
Commercial Mortgage [Member] | Doubtful [Member]    
Financing Receivable, Recorded Investment [Line Items]    
Total Loans