XML 87 R77.htm IDEA: XBRL DOCUMENT v3.24.0.1

Loans (Performance of Loans that are Modified to Borrowers Experiencing Financial Difficulty) (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	$ 935
Extended Maturity [Member]
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	935
Extended Maturity [Member] | Current
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	611
Extended Maturity [Member] | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Extended Maturity [Member] | 90+ Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	324
Commercial Business [Member]
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Commercial Business [Member] | Extended Maturity [Member]
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Commercial Business [Member] | Extended Maturity [Member] | Current
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Commercial Business [Member] | Extended Maturity [Member] | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Commercial Business [Member] | Extended Maturity [Member] | 90+ Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Commercial Mortgage [Member]
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Commercial Mortgage [Member] | Extended Maturity [Member]
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Commercial Mortgage [Member] | Extended Maturity [Member] | Current
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Commercial Mortgage [Member] | Extended Maturity [Member] | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Commercial Mortgage [Member] | Extended Maturity [Member] | 90+ Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Residential Real Estate Loans [Member]
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	935
Residential Real Estate Loans [Member] | Extended Maturity [Member]
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	935
Residential Real Estate Loans [Member] | Extended Maturity [Member] | Current
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Residential Real Estate Loans [Member] | Extended Maturity [Member] | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Residential Real Estate Loans [Member] | Extended Maturity [Member] | 90+ Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Residential Real Estate Lines [Member]
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Residential Real Estate Lines [Member] | Extended Maturity [Member]
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Residential Real Estate Lines [Member] | Extended Maturity [Member] | Current
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	611
Residential Real Estate Lines [Member] | Extended Maturity [Member] | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Residential Real Estate Lines [Member] | Extended Maturity [Member] | 90+ Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	324
Consumer Indirect [Member] | Extended Maturity [Member] | Current
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Consumer Indirect [Member] | Extended Maturity [Member] | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Consumer Indirect [Member] | Extended Maturity [Member] | 90+ Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Other Consumer [Member]
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Other Consumer [Member] | Extended Maturity [Member]
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Other Consumer [Member] | Extended Maturity [Member] | Current
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Other Consumer [Member] | Extended Maturity [Member] | 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	0
Other Consumer [Member] | Extended Maturity [Member] | 90+ Days Past Due
Financing Receivable, Modified [Line Items]
Financing receivables, amortized cost basis	$ 0

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-42

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-42

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481933/310-10-55-12A

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-33

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-34

+ Details

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amortized cost, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement and net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(5)(b))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479617/946-210-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481990/310-10-45-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-5

+ Details

Name:	us-gaap_NotesReceivableNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=fisi_CurrentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=fisi_FinancingReceivables30To89DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fisi_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fisi_ConsumerIndirectMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: