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Loans (Narrative) (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
loan
Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans held for sale $ 755,000us-gaap_LoansReceivableHeldForSaleNet $ 3,381,000us-gaap_LoansReceivableHeldForSaleNet  
Loans, related parties 11,900,000us-gaap_LoansAndLeasesReceivableRelatedParties 2,900,000us-gaap_LoansAndLeasesReceivableRelatedParties  
Loans, related parties, new borrowings 10,000,000us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions    
Loans, related parties, repayments and other reductions 1,000,000us-gaap_LoansAndLeasesReceivableRelatedPartiesCollections    
Past due greater than 90 days 8,000us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue 6,000us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue  
Estimated interest income 527,000us-gaap_InterestIncomeOperating 531,000us-gaap_InterestIncomeOperating 555,000us-gaap_InterestIncomeOperating
Number of TDR loans in default   1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1  
Days past due when loans are generally placed on nonaccrual status, in days 90 days    
Days Past Due When Loans Are Generally Placed On Nonaccrual Status, In Days 90 days    
Consumer Overdrafts [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Past due greater than 90 days $ 8,000us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fisi_ConsumerOverdraftsMember		 $ 6,000us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fisi_ConsumerOverdraftsMember