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Business Combinations (Narrative) (Details) (USD $) | 0 Months Ended | 3 Months Ended |
--- | --- | --- | ---
Aug. 01, 2014 | Mar. 31, 2015 | Apr. 15, 2015
Scott Danahy Naylon Co., Inc. [Member] |  |  |
Significant Acquisitions and Disposals [Line Items] |  |  |
Consideration including both cash and stock | $ 16,900,000us-gaap_BusinessCombinationConsiderationTransferredIncludingEquityInterestInAcquireeHeldPriorToCombination1 / us-gaap_BusinessAcquisitionAxis = fisi_ScottDanahyNaylonCo.Inc.Member |  |
Contingent future payments | 3,400,000us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_BusinessAcquisitionAxis = fisi_ScottDanahyNaylonCo.Inc.Member |  |
Estimated Fair Value Measurement Period, Number Of Years |  | 1 year |
Contingent consideration | 3,227,000us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred / us-gaap_BusinessAcquisitionAxis = fisi_ScottDanahyNaylonCo.Inc.Member |  |
Identified intangible assets | 6,640,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = fisi_ScottDanahyNaylonCo.Inc.Member |  |
Goodwill resulting from acquisition | 12,617,000us-gaap_GoodwillAcquiredDuringPeriod / us-gaap_BusinessAcquisitionAxis = fisi_ScottDanahyNaylonCo.Inc.Member |  |
Subsequent Event [Member] |  |  |
Significant Acquisitions and Disposals [Line Items] |  |  |
Principal amount |  |  | $ 40,000,000us-gaap_DebtInstrumentFaceAmount / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

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