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Summary Of Significant Accounting Policies (Summary Of Supplemental Cash Flow Information) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Summary Of Significant Accounting Policies [Abstract] |  |  |
Interest expense | $ 6,826 us-gaap_InterestPaidNet | $ 7,750 us-gaap_InterestPaidNet | $ 10,438 us-gaap_InterestPaidNet
Income taxes | 13,665 us-gaap_IncomeTaxesPaidNet | 8,095 us-gaap_IncomeTaxesPaidNet | 4,014 us-gaap_IncomeTaxesPaidNet
Real estate and other assets acquired in settlement of loans | 394 us-gaap_TransferToOtherRealEstate | 726 us-gaap_TransferToOtherRealEstate | 322 us-gaap_TransferToOtherRealEstate
Accrued and declared unpaid dividends | 3,177 fisi_DividendsAccruedAndDeclaredButNotYetPaid | 2,981 fisi_DividendsAccruedAndDeclaredButNotYetPaid | 2,562 fisi_DividendsAccruedAndDeclaredButNotYetPaid
Increase (decrease) in net unsettled security purchases | 568 fisi_IncreaseInNetUnsettledSecurityTransactions | (51,112)fisi_IncreaseInNetUnsettledSecurityTransactions | 51,135 fisi_IncreaseInNetUnsettledSecurityTransactions
Securities transferred from available for sale to held to maturity | 12,802 fisi_SecuritiesTransferredFromAvailableForSaleToHeldToMaturity | 227,330 fisi_SecuritiesTransferredFromAvailableForSaleToHeldToMaturity |
Loans transferred from held for sale to held for investment | 853 fisi_LoansTransferredFromHeldForSaleToHeldForInvestment | 3,727 fisi_LoansTransferredFromHeldForSaleToHeldForInvestment |
Common stock issued for acquisition | 5,400 us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable |  |
Loans and other non-cash assets, excluding goodwill and core deposit intangible asset | 1,007 us-gaap_NoncashOrPartNoncashAcquisitionValueOfAssetsAcquired1 |  | 77,912 us-gaap_NoncashOrPartNoncashAcquisitionValueOfAssetsAcquired1
Deposits and other liabilities | $ 1,112 us-gaap_NoncashOrPartNoncashAcquisitionValueOfLiabilitiesAssumed1 |  | $ 287,331 us-gaap_NoncashOrPartNoncashAcquisitionValueOfLiabilitiesAssumed1

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- Definition Loans transferred from held for sale to held for investment. + References No definition available. + Details
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- Definition Amount of equity interests of the acquirer, including instruments or interests issued or issuable in consideration for the business combination. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6613-128477 + Details
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