XML 97 R88.htm IDEA: XBRL DOCUMENT
v3.23.1

Commitments and Contingencies (Schedule of Credit Loss Expense Benefit for Unfunded Commitments) (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2023 | Mar. 31, 2022
Unfunded Loan Commitment [Member] | Provision For Credit Losses [Member] |  |
Commitments And Contingencies [Line Items] |  |
Credit loss expense for unfunded commitments | $ 11 | $ 241

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

X
- 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

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

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