XML 99 R90.htm IDEA: XBRL DOCUMENT v3.23.3

Commitments and Contingencies (Schedule of Credit Loss Expense Benefit for Unfunded Commitments) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Unfunded Loan Commitment [Member] | Provision For Credit Losses [Member]
Commitments And Contingencies [Line Items]
Credit loss (benefit) expense for unfunded commitments	$ (426)	$ 507	$ (129)	$ 867

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: