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