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Loans Held For Sale And Loan Servicing Rights (Narrative) (Details) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Loans Held For Sale And Loan Servicing Rights [Abstract]    
Loans held for sale $ 755,000us-gaap_LoansReceivableHeldForSaleNet $ 3,381,000us-gaap_LoansReceivableHeldForSaleNet
Residential real estate mortgages serviced for others 215,200,000fisi_ResidentialRealEstateMortgagesServicedForOthers 237,900,000fisi_ResidentialRealEstateMortgagesServicedForOthers
Escrow and other custodial funds $ 4,600,000us-gaap_EscrowDeposit $ 4,900,000us-gaap_EscrowDeposit