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Loans (Recorded Investment By Loan Class In Current And Nonaccrual Loans) (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
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Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |
30-59 Days Past Due | $ 3,808 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue | $ 4,905 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue |
60-89 Days Past Due | 517 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue | 890 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue |
Greater Than 90 Days | 8 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue | 6 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue |
Total Past Due | 4,333 us-gaap_FinancingReceivableRecordedInvestmentPastDue | 5,801 us-gaap_FinancingReceivableRecordedInvestmentPastDue |
Nonaccrual | 10,145 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus | 16,616 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus |
Current | 1,866,593 us-gaap_FinancingReceivableRecordedInvestmentCurrent | 1,779,688 us-gaap_FinancingReceivableRecordedInvestmentCurrent |
Total loans | 1,881,071 us-gaap_NotesReceivableGross | 1,802,105 us-gaap_NotesReceivableGross | 1,675,453 us-gaap_NotesReceivableGross
Commercial Business [Member] |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |
30-59 Days Past Due | 28 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember | 558 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember |
60-89 Days Past Due |  | 199 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember |
Total Past Due | 28 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember | 757 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember |
Nonaccrual | 4,288 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember | 3,474 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember |
Current | 263,061 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember | 261,520 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember |
Total loans | 267,377 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember | 265,751 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember | 258,706 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_CommercialAndIndustrialMember
Commercial Mortgage [Member] |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |
30-59 Days Past Due | 83 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | 800 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember |
Total Past Due | 83 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | 800 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember |
Nonaccrual | 3,020 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | 9,663 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember |
Current | 473,304 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | 459,849 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember |
Total loans | 476,407 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | 470,312 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember | 414,282 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstateMember
Residential Mortgage [Member] |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |
30-59 Days Past Due | 321 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember | 542 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember |
Total Past Due | 321 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember | 542 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember |
Nonaccrual | 1,194 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember | 1,078 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember |
Current | 98,726 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember | 111,481 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember |
Total loans | 100,241 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember | 113,101 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember | 133,341 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ResidentialMortgageMember
Home Equity [Member] |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |
30-59 Days Past Due | 799 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember | 750 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember |
60-89 Days Past Due | 67 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember | 143 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember |
Total Past Due | 866 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember | 893 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember |
Nonaccrual | 463 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember | 925 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember |
Current | 378,445 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember | 318,840 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember |
Total loans | 379,774 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember | 320,658 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember | 282,503 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_HomeEquityMember
Consumer Indirect [Member] |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |
30-59 Days Past Due | 2,429 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerIndirectMember | 2,129 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerIndirectMember |
60-89 Days Past Due | 402 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerIndirectMember | 476 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerIndirectMember |
Total Past Due | 2,831 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerIndirectMember | 2,605 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerIndirectMember |
Nonaccrual | 1,169 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerIndirectMember | 1,471 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerIndirectMember |
Current | 632,357 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerIndirectMember | 605,314 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerIndirectMember |
Total loans | 636,357 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerIndirectMember | 609,390 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerIndirectMember | 559,964 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = fisi_ConsumerIndirectMember
Other Consumer [Member] |  |  |
Financing Receivable, Recorded Investment, Past Due [Line Items] |  |  |
30-59 Days Past Due | 148 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember | 126 us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember |
60-89 Days Past Due | 48 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember | 72 us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember |
Greater Than 90 Days | 8 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember | 6 us-gaap_FinancingReceivableRecordedInvestmentEqualToGreaterThan90DaysPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember |
Total Past Due | 204 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember | 204 us-gaap_FinancingReceivableRecordedInvestmentPastDue / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember |
Nonaccrual | 11 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember | 5 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember |
Current | 20,700 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember | 22,684 us-gaap_FinancingReceivableRecordedInvestmentCurrent / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember |
Total loans | $ 20,915 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember | $ 22,893 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember | $ 26,657 us-gaap_NotesReceivableGross / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_ConsumerOtherFinancingReceivableMember

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- Definition Financing receivables that are less than 60 days past due but more than 29 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 Financing receivables that are less than 90 days past due but more than 59 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 Financing receivables that are current. + 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 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 Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=28368275&loc=d3e5111-111524 + Details
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- Definition Recorded Investment in financing receivables that are past due at the balance sheet date. + 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 representing an agreement for an unconditional promise by the maker to pay the entity (holder) a definite sum of money at a future date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. Excludes amounts related to receivables held-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.10(3)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 10 -Subparagraph 3 -Article 9 + Details
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