XML 68 R61.htm IDEA: XBRL DOCUMENT
v3.24.2.u1

Leases (Summary of Classification of Right of Use Assets and Lease Liabilities) (Details) - USD ($) $ in Thousands | Jun. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Operating Lease Right of Use Assets: |  |
Gross carrying amount | $ 39,328 | $ 38,684
Accumulated amortization | (8,052) | (7,160)
Net book value | $ 31,276 | $ 31,524
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List] | Other assets | Other assets
Operating Lease Liabilities: |  |
Operating lease right of use obligations | $ 33,605 | $ 33,788
Operating Lease, Liability, Statement of Financial Position [Extensible List] | Other liabilities | Other liabilities

X
- Definition Operating lease liabilities. + References No definition available. + Details
Name: | fisi_OperatingLeaseLiabilitiesAbstract
Namespace Prefix: | fisi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Operating lease right of use asset accumulated amortization. + References No definition available. + Details
Name: | fisi_OperatingLeaseRightOfUseAssetAccumulatedAmortization
Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Operating lease right of use asset gross. + References No definition available. + Details
Name: | fisi_OperatingLeaseRightOfUseAssetGross
Namespace Prefix: | fisi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Operating lease right of use assets. + References No definition available. + Details
Name: | fisi_OperatingLeaseRightOfUseAssetsAbstract
Namespace Prefix: | fisi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Present value of lessee's discounted obligation for lease payments from operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479041/842-20-45-1 + Details
Name: | us-gaap_OperatingLeaseLiability
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Indicates line item in statement of financial position that includes operating lease liability. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479041/842-20-45-2 + Details
Name: | us-gaap_OperatingLeaseLiabilityStatementOfFinancialPositionExtensibleList
Namespace Prefix: | us-gaap_
Data Type: | enum2:enumerationSetItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of lessee's right to use underlying asset under operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479041/842-20-45-1 + Details
Name: | us-gaap_OperatingLeaseRightOfUseAsset
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Indicates line item in statement of financial position that includes operating lease right-of-use asset. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479041/842-20-45-2 + Details
Name: | us-gaap_OperatingLeaseRightOfUseAssetStatementOfFinancialPositionExtensibleList
Namespace Prefix: | us-gaap_
Data Type: | enum2:enumerationSetItemType
Balance Type: | na
Period Type: | instant