XML 51 R44.htm IDEA: XBRL DOCUMENT v3.25.1

Basis of Presentation and Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Thousands		3 Months Ended
	Apr. 01, 2024	Mar. 31, 2025	Mar. 31, 2024
Basis of Presentation and Significant Accounting Policies [Line Items]
Net loss on sale of other assets		$ 0	$ (13)
SDN [Member]
Basis of Presentation and Significant Accounting Policies [Line Items]
Proceeds from divestiture of interest in consolidated subsidiaries	$ 27,000
Net loss on sale of other assets	$ 13,700

X

- Definition

Basis of presentation and significant accounting policies

+ References

No definition available.

+ Details

Name:	fisi_BasisOfPresentationAndSignificantAccountingPoliciesLineItems
Namespace Prefix:	fisi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of gain (loss) on sale or disposal of other assets.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

+ Details

Name:	us-gaap_GainLossOnSaleOfOtherAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The cash inflow associated with the deconsolidation of a previously consolidated subsidiary or the sale of investment in consolidated subsidiaries (generally greater than 50 percent).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-12

+ Details

Name:	us-gaap_ProceedsFromDivestitureOfInterestInConsolidatedSubsidiaries
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=fisi_SDNInsuranceAgencyLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: