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Commitments and Contingencies (Schedule of Credit Loss (Benefit) for Unfunded Commitments) (Details) - USD ($) $ in Thousands	3 Months Ended
	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)

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: