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Fair Value Measurements (Additional Quantitative Information About Assets Measured At Fair Value On A Recurring And Non-Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015
Collateral Dependent Impaired Loans [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Assets at fair value | $ 3,391us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = fisi_CollateralDependentImpairedFinancingReceivableMember | [1]
Discount rate | 4.40%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByAssetClassAxis = fisi_CollateralDependentImpairedFinancingReceivableMember | [2]
Risk premium rate | 10.00%fisi_FairValueInputsRiskPremiumRate / us-gaap_FairValueByAssetClassAxis = fisi_CollateralDependentImpairedFinancingReceivableMember | [2]
Loan Servicing Rights [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Assets at fair value | 1,325us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = fisi_LoanServicingRightsMember |
Discount rate | 4.80%us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByAssetClassAxis = fisi_LoanServicingRightsMember | [2]
Constant prepayment rate | 15.60%us-gaap_FairValueInputsPrepaymentRate / us-gaap_FairValueByAssetClassAxis = fisi_LoanServicingRightsMember | [2]
Other Real Estate Owned [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Assets at fair value | $ 139us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = fisi_OtherRealEstateOwnedOtherReceivableMember | [1]
Minimum [Member] | Collateral Dependent Impaired Loans [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Appraisal adjustments | 0.00%us-gaap_FairValueInputsComparabilityAdjustments / us-gaap_FairValueByAssetClassAxis = fisi_CollateralDependentImpairedFinancingReceivableMember / us-gaap_RangeAxis = us-gaap_MinimumMember | [3]
Minimum [Member] | Other Real Estate Owned [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Appraisal adjustments | 19.00%us-gaap_FairValueInputsComparabilityAdjustments / us-gaap_FairValueByAssetClassAxis = fisi_OtherRealEstateOwnedOtherReceivableMember / us-gaap_RangeAxis = us-gaap_MinimumMember | [3]
Maximum [Member] | Collateral Dependent Impaired Loans [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Appraisal adjustments | 100.00%us-gaap_FairValueInputsComparabilityAdjustments / us-gaap_FairValueByAssetClassAxis = fisi_CollateralDependentImpairedFinancingReceivableMember / us-gaap_RangeAxis = us-gaap_MaximumMember | [3]
Maximum [Member] | Other Real Estate Owned [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Appraisal adjustments | 43.00%us-gaap_FairValueInputsComparabilityAdjustments / us-gaap_FairValueByAssetClassAxis = fisi_OtherRealEstateOwnedOtherReceivableMember / us-gaap_RangeAxis = us-gaap_MaximumMember | [3]
[1] | Fair value is generally determined through independent appraisals of the underlying collateral, which generally include various Level 3 inputs which are not identifiable.
[2] | Weighted averages.
[3] | Appraisals may be adjusted by management for qualitative factors such as economic conditions and estimated liquidation expenses.

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- Definition Rate of return in excess of the risk-free rate assumption, used as an input to measure fair value. + References No definition available. + Details
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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Percentage of adjustment to the observed price for a similar asset or liability when an identical transaction does not exist, used as an input to measure fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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