XML 65 R58.htm IDEA: XBRL DOCUMENT
v3.25.3

Loans (Commercial Loan Portfolio Categorized by Internally Assigned Asset Classification) (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Commercial Business [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | $ 100,205 | $ 123,481
Prior Fiscal Year 1 | 98,165 | 96,369
Prior Fiscal Year 2 | 82,974 | 75,025
Prior Fiscal Year 3 | 73,260 | 43,663
Prior Fiscal Year 4 | 35,725 | 20,775
Prior | 27,812 | 21,443
Revolving Loans Amortized Cost Basis | 322,462 | 284,565
Revolving Loans Converted to Term | 0 | 0
Total | 740,603 | 665,321
Commercial Business [Member] | Uncriticized [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 100,179 | 121,094
Prior Fiscal Year 1 | 95,543 | 90,618
Prior Fiscal Year 2 | 78,534 | 69,669
Prior Fiscal Year 3 | 70,148 | 43,566
Prior Fiscal Year 4 | 35,693 | 20,745
Prior | 26,945 | 19,409
Revolving Loans Amortized Cost Basis | 307,062 | 267,186
Revolving Loans Converted to Term | 0 | 0
Total | 714,104 | 632,287
Commercial Business [Member] | Special Mention [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 26 | 2,218
Prior Fiscal Year 1 | 2,443 | 4,814
Prior Fiscal Year 2 | 4,119 | 297
Prior Fiscal Year 3 | 5 | 57
Prior Fiscal Year 4 | 11 | 8
Prior | 522 | 1,678
Revolving Loans Amortized Cost Basis | 7,253 | 9,297
Revolving Loans Converted to Term | 0 | 0
Total | 14,379 | 18,369
Commercial Business [Member] | Substandard [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 169
Prior Fiscal Year 1 | 117 | 937
Prior Fiscal Year 2 | 321 | 20
Prior Fiscal Year 3 | 8 | 19
Prior Fiscal Year 4 | 0 | 22
Prior | 173 | 184
Revolving Loans Amortized Cost Basis | 7,860 | 7,789
Revolving Loans Converted to Term | 0 | 0
Total | 8,479 | 9,140
Commercial Business [Member] | Doubtful [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 62 | 0
Prior Fiscal Year 2 | 0 | 5,039
Prior Fiscal Year 3 | 3,099 | 21
Prior Fiscal Year 4 | 21 | 0
Prior | 172 | 172
Revolving Loans Amortized Cost Basis | 287 | 293
Revolving Loans Converted to Term | 0 | 0
Total | 3,641 | 5,525
Commercial Business [Member] | Current Period Gross Write-offs [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 75 | 5
Prior Fiscal Year 2 | 0 | 0
Prior Fiscal Year 3 | 1,953 | 20
Prior Fiscal Year 4 | 14 | 0
Prior | 173 | 274
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 2,215 | 299
Commercial Mortgage [Member] | Construction [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 69,317 | 52,470
Prior Fiscal Year 1 | 23,744 | 176,379
Prior Fiscal Year 2 | 105,847 | 311,904
Prior Fiscal Year 3 | 134,494 | 25,497
Prior Fiscal Year 4 | 151,495 | 341
Prior | 107,737 | 16,028
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 592,634 | 582,619
Commercial Mortgage [Member] | Multifamily [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 31,139 | 26,339
Prior Fiscal Year 1 | 90,014 | 105,838
Prior Fiscal Year 2 | 150,979 | 47,549
Prior Fiscal Year 3 | 140,698 | 143,994
Prior Fiscal Year 4 | 22,211 | 72,494
Prior | 5,993 | 74,740
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 441,034 | 470,954
Commercial Mortgage [Member] | Non-Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 105,314 | 83,677
Prior Fiscal Year 1 | 86,403 | 79,332
Prior Fiscal Year 2 | 81,673 | 245,911
Prior Fiscal Year 3 | 238,220 | 104,074
Prior Fiscal Year 4 | 78,024 | 82,368
Prior | 304,250 | 262,625
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 893,884 | 857,987
Commercial Mortgage [Member] | Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 26,431 | 67,060
Prior Fiscal Year 1 | 69,091 | 41,274
Prior Fiscal Year 2 | 23,805 | 49,192
Prior Fiscal Year 3 | 63,734 | 34,049
Prior Fiscal Year 4 | 37,988 | 44,020
Prior | 100,506 | 52,441
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 321,555 | 288,036
Commercial Mortgage [Member] | Uncriticized [Member] | Construction [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 31,139 | 52,470
Prior Fiscal Year 1 | 90,014 | 175,559
Prior Fiscal Year 2 | 144,738 | 311,182
Prior Fiscal Year 3 | 137,431 | 3,753
Prior Fiscal Year 4 | 0 | 341
Prior | 5,993 | 6,516
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 409,315 | 549,821
Commercial Mortgage [Member] | Uncriticized [Member] | Multifamily [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 69,317 | 26,339
Prior Fiscal Year 1 | 23,744 | 105,838
Prior Fiscal Year 2 | 105,847 | 43,924
Prior Fiscal Year 3 | 129,967 | 143,994
Prior Fiscal Year 4 | 146,912 | 65,206
Prior | 99,123 | 67,715
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 574,910 | 453,016
Commercial Mortgage [Member] | Uncriticized [Member] | Non-Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 105,314 | 83,677
Prior Fiscal Year 1 | 86,403 | 66,338
Prior Fiscal Year 2 | 68,679 | 245,618
Prior Fiscal Year 3 | 238,220 | 99,175
Prior Fiscal Year 4 | 78,024 | 82,368
Prior | 298,392 | 250,316
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 875,032 | 827,492
Commercial Mortgage [Member] | Uncriticized [Member] | Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 24,181 | 67,060
Prior Fiscal Year 1 | 69,091 | 41,274
Prior Fiscal Year 2 | 23,805 | 48,517
Prior Fiscal Year 3 | 63,734 | 34,049
Prior Fiscal Year 4 | 37,786 | 43,528
Prior | 96,682 | 50,967
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 315,279 | 285,395
Commercial Mortgage [Member] | Special Mention [Member] | Construction [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 0
Prior Fiscal Year 2 | 6,241 | 722
Prior Fiscal Year 3 | 3,267 | 2,284
Prior Fiscal Year 4 | 2,417 | 0
Prior | 0 | 0
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 11,925 | 3,006
Commercial Mortgage [Member] | Special Mention [Member] | Multifamily [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year |  | 0
Prior Fiscal Year 1 |  | 0
Prior Fiscal Year 2 |  | 3,625
Prior Fiscal Year 3 |  | 0
Prior Fiscal Year 4 |  | 6,825
Prior | 7,418 | 6,234
Revolving Loans Amortized Cost Basis |  | 0
Revolving Loans Converted to Term |  | 0
Total | 12,317 | 16,684
Commercial Mortgage [Member] | Special Mention [Member] | Non-Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 0
Prior Fiscal Year 2 | 0 | 0
Prior Fiscal Year 3 | 316 | 4,691
Prior Fiscal Year 4 | 4,583 | 0
Prior | 5,683 | 7,829
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 5,683 | 12,520
Commercial Mortgage [Member] | Special Mention [Member] | Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 2,250 | 0
Prior Fiscal Year 1 | 0 | 0
Prior Fiscal Year 2 | 0 | 451
Prior Fiscal Year 3 | 0 | 0
Prior Fiscal Year 4 | 0 | 126
Prior | 1,976 | 1,110
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 4,226 | 1,687
Commercial Mortgage [Member] | Substandard [Member] | Construction [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 0
Prior Fiscal Year 2 | 0 | 0
Prior Fiscal Year 3 | 0 | 0
Prior Fiscal Year 4 | 0 | 0
Prior | 0 | 9,512
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 0 | 9,512
Commercial Mortgage [Member] | Substandard [Member] | Multifamily [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 0
Prior Fiscal Year 2 | 0 | 0
Prior Fiscal Year 3 | 4,211 | 0
Prior Fiscal Year 4 | 0 | 463
Prior | 656 | 791
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 4,867 | 1,254
Commercial Mortgage [Member] | Substandard [Member] | Non-Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 12,994
Prior Fiscal Year 2 | 12,994 | 0
Prior Fiscal Year 3 | 0 | 208
Prior Fiscal Year 4 | 0 | 0
Prior | 175 | 0
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 13,169 | 13,202
Commercial Mortgage [Member] | Substandard [Member] | Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 0
Prior Fiscal Year 2 | 0 | 224
Prior Fiscal Year 3 | 0 | 0
Prior Fiscal Year 4 | 202 | 366
Prior | 746 | 10
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 948 | 600
Commercial Mortgage [Member] | Doubtful [Member] | Construction [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 820
Prior Fiscal Year 2 | 0 | 0
Prior Fiscal Year 3 | 0 | 19,460
Prior Fiscal Year 4 | 19,794 | 0
Prior | 0 | 0
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 19,794 | 20,280
Commercial Mortgage [Member] | Doubtful [Member] | Multifamily [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 0
Prior Fiscal Year 2 | 0 | 0
Prior Fiscal Year 3 | 0 | 0
Prior Fiscal Year 4 | 0 | 0
Prior | 540 | 0
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 540 | 0
Commercial Mortgage [Member] | Doubtful [Member] | Non-Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 0
Prior Fiscal Year 2 | 0 | 293
Prior Fiscal Year 3 | 0 | 0
Prior Fiscal Year 4 | 0 | 0
Prior | 0 | 4,480
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 0 | 4,773
Commercial Mortgage [Member] | Doubtful [Member] | Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 0
Prior Fiscal Year 2 | 0 | 0
Prior Fiscal Year 3 | 0 | 0
Prior Fiscal Year 4 | 0 | 0
Prior | 1,102 | 354
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 1,102 | 354
Commercial Mortgage [Member] | Current Period Gross Write-offs [Member] | Construction [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 0
Prior Fiscal Year 2 | 0 | 0
Prior Fiscal Year 3 | 0 | 0
Prior Fiscal Year 4 | 0 | 0
Prior | 0 | 0
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 0 | 0
Commercial Mortgage [Member] | Current Period Gross Write-offs [Member] | Multifamily [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 0
Prior Fiscal Year 2 | 0 | 0
Prior Fiscal Year 3 | 0 | 13
Prior Fiscal Year 4 | 0 | 0
Prior | 0 | 0
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 0 | 13
Commercial Mortgage [Member] | Current Period Gross Write-offs [Member] | Non-Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 0
Prior Fiscal Year 2 | 0 | 0
Prior Fiscal Year 3 | 0 | 0
Prior Fiscal Year 4 | 0 | 0
Prior | 597 | 0
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | 597 | 0
Commercial Mortgage [Member] | Current Period Gross Write-offs [Member] | Owner Occupied [Member] |  |
Financing Receivable Recorded Investment [Line Items] |  |
Current Fiscal Year | 0 | 0
Prior Fiscal Year 1 | 0 | 0
Prior Fiscal Year 2 | 0 | 0
Prior Fiscal Year 3 | 0 | 0
Prior Fiscal Year 4 | 0 | 0
Prior | 0 | 0
Revolving Loans Amortized Cost Basis | 0 | 0
Revolving Loans Converted to Term | 0 | 0
Total | $ 0 | $ 0

X
- Definition Financing receivable after deferred income offset amount. + References No definition available. + Details
Name: | fisi_FinancingReceivableAfterDeferredIncomeOffsetAmount
Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of financing receivable originated more than five years prior to current fiscal year. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6 + Details
Name: | us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of financing receivable originated four years prior to current fiscal year. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6 + Details
Name: | us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of financing receivable originated in current fiscal year. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6 + Details
Name: | us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of financing receivable originated in fiscal year prior to current fiscal year. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6 + Details
Name: | us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of financing receivable originated three years prior to current fiscal year. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6 + Details
Name: | us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of financing receivable originated two years prior to current fiscal year. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6 + Details
Name: | us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (a)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (d)(4) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 5: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (d)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (d)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (d)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 8: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (b)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 9: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Subparagraph (g) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-15 Reference 10: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-5 Reference 11: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-15 Reference 12: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 13: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (c)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 14: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (e)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 15: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (e)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 16: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (b)(4) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 17: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (b)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 18: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-12 Reference 19: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-15 Reference 20: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-15 Reference 21: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-15 Reference 22: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 29 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481962/310-10-50-29 Reference 23: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 + Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amortized cost of financing receivable that can be withdrawn, repaid, and redrawn. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6 + Details
Name: | us-gaap_FinancingReceivableRevolving
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amortized cost of revolving financing receivable converted to term loan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 79 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479294/326-20-55-79 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6A -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-6A + Details
Name: | us-gaap_FinancingReceivableRevolvingConvertedToTermLoan
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fisi_CommercialAndIndustrialMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_PassMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_DoubtfulMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=fisi_CurrentPeriodGrossWriteOffsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=fisi_ConstructionMortgageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=fisi_MultifamilyMortgageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=fisi_NonOwnerOccupiedMortgageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=fisi_OwnerOccupiedMortgageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |