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v2.4.1.9

Loans (Narrative) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 loan | Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Loans held for sale | $ 755,000 us-gaap_LoansReceivableHeldForSaleNet | $ 3,381,000 us-gaap_LoansReceivableHeldForSaleNet |
Loans, related parties | 11,900,000 us-gaap_LoansAndLeasesReceivableRelatedParties | 2,900,000 us-gaap_LoansAndLeasesReceivableRelatedParties |
Loans, related parties, new borrowings | 10,000,000 us-gaap_LoansAndLeasesReceivableRelatedPartiesAdditions |  |
Loans, related parties, repayments and other reductions | 1,000,000 us-gaap_LoansAndLeasesReceivableRelatedPartiesCollections |  |
Past due greater than 90 days | 8,000 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue | 6,000 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue |
Estimated interest income | 527,000 us-gaap_InterestIncomeOperating | 531,000 us-gaap_InterestIncomeOperating | 555,000 us-gaap_InterestIncomeOperating
Number of TDR loans in default |  | 1 us-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 | $ 8,000 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerOverdraftsMember | $ 6,000 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerOverdraftsMember |

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- Definition Days Past Due When Loans Are Generally Placed On Nonaccrual Status, In Days + References No definition available. + Details
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- Definition Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=28368275&loc=SL6953682-111524 + Details
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- Definition Financing receivables that are equal to or greater than 90 days past due. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=28368275&loc=SL6953401-111524 + Details
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- Definition Amount of operating interest income, including, but not limited to, amortization and accretion of premiums and discounts on securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1(e)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
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- Definition For an unclassified balance sheet, reflects the carrying amount of unpaid loan amounts due from related parties at the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(e)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph e -Article 9 + Details
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- Definition Reflects the amount of new advances made to related parties in the form of loan and lease receivables. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(e)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph e -Article 9 + Details
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- Definition Reflects the amount of collections received from related parties to reduce the carrying amounts of loans and leases receivable due from them. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(e)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph e -Article 9 + Details
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- Definition Amount of loans held for sale, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Subparagraph a -Article 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(a)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 35 -Paragraph 49 -URI http://asc.fasb.org/extlink&oid=25442802&loc=d3e3084-111518 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 310 -Section 30 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6490209&loc=d3e48214-111002 + Details
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