Certificate of deposit penalty fees for withdrawals.

No definition available.

Name:	fisi_CertificateOfDepositPenaltyFeesForWithdrawals
Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Estimated amount of relief fund.

+ References

No definition available.

+ Details

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Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Loans and leases receivable net reported forgiveness amount.

+ References

No definition available.

+ Details

Name:	fisi_LoansAndLeasesReceivableNetReportedForgivenessAmount
Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Number of customers.

+ References

No definition available.

+ Details

Name:	fisi_NumberOfCustomers
Namespace Prefix:	fisi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of designated derivative instrument.

+ References

No definition available.

+ Details

Name:	fisi_NumberOfDesignatedDerivativeInstrument
Namespace Prefix:	fisi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Offering unsecured personal loans, amount.

+ References

No definition available.

+ Details

Name:	fisi_OfferingUnsecuredPersonalLoansAmount
Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Offering unsecured personal loans, percentage.

+ References

No definition available.

+ Details

Name:	fisi_OfferingUnsecuredPersonalLoansPercentage
Namespace Prefix:	fisi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Offering unsecured personal loans, period.

+ References

No definition available.

+ Details

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Namespace Prefix:	fisi_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of loans with active payment deferrals.

+ References

No definition available.

+ Details

Name:	fisi_PercentageOfLoansWithActivePaymentDeferrals
Namespace Prefix:	fisi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Significant Accounting Policies [Line Items]

+ References

No definition available.

+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

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-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Description of the interest rate as being fixed or variable, and, if variable, identification of the index or rate on which the interest rate is based and the number of points or percentage added to that index or rate to set the rate, and other pertinent information, such as frequency of rate resets.

+ References

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
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-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fisi_ResidentialRealEstateLoansAndLinesMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
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- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fisi_ConsumerIndirectMember
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Balance Type:
Period Type:

- Details

Name:	us-gaap_UnusualOrInfrequentItemAxis=fisi_CoronavirusAidReliefAndEconomicSecurityActMember
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- Details

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Basis of Presentation and Summary of Significant Accounting Policies (Narrative) (Details) $ in Thousands		3 Months Ended	4 Months Ended
	Mar. 27, 2020 USD ($)	Mar. 31, 2021 USD ($) DerivativeInstrument	Jul. 09, 2020 USD ($)	Dec. 31, 2020 USD ($) Customer	Dec. 27, 2020 USD ($)	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Loans		$ 3,604,558		$ 3,542,718
Offering unsecured personal loans, percentage			2.95%
Subordinated Notes [Member]
Significant Accounting Policies [Line Items]
Number of designated derivative instrument \| DerivativeInstrument		1
Debt instrument issued amount						$ 40,000
Subordinated Notes [Member] \| LIBOR [Member]
Significant Accounting Policies [Line Items]
Debt instrument, fixed rate of interest						6.00%
Debt instrument, interest rate terms		three-month LIBOR plus 3.944%
Debt instrument, basis spread on variable rate		3.944%
Commercial Loan and Mortgage Loans [Member]
Significant Accounting Policies [Line Items]
Percentage of loans with active payment deferrals		4.00%
Residential Real Estate Loans and Lines [Member]
Significant Accounting Policies [Line Items]
Percentage of loans with active payment deferrals		1.00%
Maximum [Member]
Significant Accounting Policies [Line Items]
Certificate of deposit penalty fees for withdrawals			$ 20,000
Offering unsecured personal loans, amount			$ 5,000
Offering unsecured personal loans, period			60 months
Maximum [Member] \| Consumer Indirect [Member]
Significant Accounting Policies [Line Items]
Percentage of loans with active payment deferrals		1.00%
CARES Act [Member]
Significant Accounting Policies [Line Items]
Estimated amount of relief fund	$ 2,200,000,000
Paycheck Protection Program [Member]
Significant Accounting Policies [Line Items]
Loans		$ 262,100		253,100	$ 284,000,000
Loans forgiveness amount		87,000
Paycheck Protection Program [Member] \| Minimum [Member]
Significant Accounting Policies [Line Items]
Loans				$ 270,000
Number of customers \| Customer				1,700
New Paycheck Protection Program [Member]
Significant Accounting Policies [Line Items]
Loans		$ 96,000