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v3.23.1

Loans (Financial Effect of the Modifications Made to Borrowers Experiencing Financial Difficulty) (Details) | 3 Months Ended
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Mar. 31, 2023
Residential Mortgage [Member] | Extended Maturity [Member] |
Financing Receivable, Troubled Debt Restructuring [Line Items] |
Added Weighted Average Loans Life | 10 years

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- Definition Added weighted average loans life. + References No definition available. + Details
Name: | fisi_AddedWeightedAverageLoansLife
Namespace Prefix: | fisi_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- 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 No definition available. + Details
Name: | us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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- Details
Name: | us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |