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Commitments and Contingencies (Schedule of Credit Loss (Benefit) for Unfunded Commitments) (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2025 | Mar. 31, 2024
Unfunded Loan Commitment [Member] | Provision For Credit Losses [Member] |  |
Commitments And Contingencies [Line Items] |  |
Credit loss (benefit) for unfunded commitments | $ (364) | $ (570)

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- Definition Commitments and contingencies. + References No definition available. + Details
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Namespace Prefix: | fisi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Credit loss benefit expense. + References No definition available. + Details
Name: | fisi_CreditLossBenefitExpense
Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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- Details
Name: | us-gaap_IncomeStatementLocationAxis=fisi_ProvisionForCreditLossesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |