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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
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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 + Details
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