Loans (Commercial Loan Portfolio Categorized by Internally Assigned Asset Classification) (Details) $ in Thousands	Sep. 30, 2020 USD ($)
Commercial Business [Member]
Financing Receivable Recorded Investment [Line Items]
2020	$ 357,814
2019	111,967
2018	91,362
2017	33,728
2016	12,768
Prior	19,995
Revolving Loans Amortized Cost Basis	190,501
Revolving Loans Converted to Term	0
Total	818,135
Commercial Business [Member] \| Uncriticized [Member]
Financing Receivable Recorded Investment [Line Items]
2020	356,882
2019	111,373
2018	90,038
2017	32,169
2016	12,489
Prior	19,063
Revolving Loans Amortized Cost Basis	183,654
Revolving Loans Converted to Term	0
Total	805,668
Commercial Business [Member] \| Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
2020	709
2019	108
2018	3
2017	802
2016	40
Prior	3
Revolving Loans Amortized Cost Basis	2,088
Revolving Loans Converted to Term	0
Total	3,753
Commercial Business [Member] \| Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
2020	223
2019	486
2018	1,321
2017	757
2016	239
Prior	929
Revolving Loans Amortized Cost Basis	4,759
Revolving Loans Converted to Term	0
Total	8,714
Commercial Business [Member] \| Doubtful [Member]
Financing Receivable Recorded Investment [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	0
Revolving Loans Converted to Term	0
Total	0
Commercial Mortgage [Member]
Financing Receivable Recorded Investment [Line Items]
2020	213,530
2019	283,156
2018	205,267
2017	191,696
2016	96,903
Prior	210,865
Revolving Loans Amortized Cost Basis	629
Revolving Loans Converted to Term	0
Total	1,202,046
Commercial Mortgage [Member] \| Uncriticized [Member]
Financing Receivable Recorded Investment [Line Items]
2020	213,341
2019	278,617
2018	186,224
2017	189,627
2016	96,571
Prior	199,397
Revolving Loans Amortized Cost Basis	430
Revolving Loans Converted to Term	0
Total	1,164,207
Commercial Mortgage [Member] \| Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
2020	0
2019	2,091
2018	17,133
2017	454
2016	329
Prior	1,449
Revolving Loans Amortized Cost Basis	0
Revolving Loans Converted to Term	0
Total	21,456
Commercial Mortgage [Member] \| Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
2020	189
2019	2,448
2018	1,910
2017	1,615
2016	3
Prior	10,019
Revolving Loans Amortized Cost Basis	199
Revolving Loans Converted to Term	0
Total	16,383
Commercial Mortgage [Member] \| Doubtful [Member]
Financing Receivable Recorded Investment [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	0
Revolving Loans Converted to Term	0
Total	$ 0

Financing receivable after deferred income offset amount.

No definition available.

Name:	fisi_FinancingReceivableAfterDeferredIncomeOffsetAmount
Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of financing receivable originated more than five years prior to current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922352-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919232-210447

+ Details

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

- Definition

Amount of financing receivable originated four years prior to current fiscal year.

+ References

+ Details

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

- Definition

Amount of financing receivable originated in current fiscal year.

+ References

+ Details

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

- Definition

Amount of financing receivable originated in fiscal year prior to current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919232-210447

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922352-210448

+ Details

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

- Definition

Amount of financing receivable originated three years prior to current fiscal year.

+ References

+ Details

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

- Definition

Amount of financing receivable originated two years prior to current fiscal year.

+ References

+ Details

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

- 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

No definition available.

+ Details

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

- Definition

Amount of financing receivable that can be withdrawn, repaid and redrawn.

+ References

+ Details

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

- Definition

Amount of revolving financing receivable converted to term loan.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922352-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 6A
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL120270059-210447

+ Details

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

- Details

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

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_PassMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_DoubtfulMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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