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Loans (Commercial Loan Portfolio Categorized By Internally Assigned Asset Classification) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Total Loans	$ 1,892,390us-gaap_LoansAndLeasesReceivableGrossCarryingAmount	$ 1,881,071us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Commercial Business [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	277,427us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember	267,377us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember
Commercial Business [Member] | Uncriticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	261,447us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember / us-gaap_InternalCreditAssessmentAxis = us-gaap_PassMember	250,961us-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	5,226us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember / us-gaap_InternalCreditAssessmentAxis = us-gaap_SpecialMentionMember	5,530us-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	10,754us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember / us-gaap_InternalCreditAssessmentAxis = us-gaap_SubstandardMember	10,886us-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	480,479us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember	476,407us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember
Commercial Mortgage [Member] | Uncriticized [Member]
Financing Receivable, Recorded Investment [Line Items]
Total Loans	465,383us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember / us-gaap_InternalCreditAssessmentAxis = us-gaap_PassMember	460,880us-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	5,598us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember / us-gaap_InternalCreditAssessmentAxis = us-gaap_SpecialMentionMember	5,411us-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	9,498us-gaap_LoansAndLeasesReceivableGrossCarryingAmount / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember / us-gaap_InternalCreditAssessmentAxis = us-gaap_SubstandardMember	10,116us-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

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- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522

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