Loans (Retail Loan Portfolio Categorized by Payment Status) (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Residential Real Estate Loans [Member]
Financing Receivable Recorded Investment [Line Items]
2020	$ 28,479
2019	112,348
2018	106,356
2017	83,223
2016	80,012
Prior	169,382
Revolving Loans Amortized Cost Basis	0
Revolving Loans Converted to Term	0
Total	579,800
Residential Real Estate Loans [Member] \| Performing [Member]
Financing Receivable Recorded Investment [Line Items]
2020	28,479
2019	112,239
2018	105,772
2017	83,138
2016	79,834
Prior	168,367
Revolving Loans Amortized Cost Basis	0
Revolving Loans Converted to Term	0
Total	577,829
Residential Real Estate Loans [Member] \| Non-Performing [Member]
Financing Receivable Recorded Investment [Line Items]
2020	0
2019	109
2018	584
2017	85
2016	178
Prior	1,015
Revolving Loans Amortized Cost Basis	0
Revolving Loans Converted to Term	0
Total	1,971
Residential Real Estate Lines [Member]
Financing Receivable Recorded Investment [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	91,090
Revolving Loans Converted to Term	11,023
Total	102,113
Residential Real Estate Lines [Member] \| Performing [Member]
Financing Receivable Recorded Investment [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	91,044
Revolving Loans Converted to Term	10,926
Total	101,970
Residential Real Estate Lines [Member] \| Non-Performing [Member]
Financing Receivable Recorded Investment [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans Amortized Cost Basis	46
Revolving Loans Converted to Term	97
Total	143
Consumer Indirect [Member]
Financing Receivable Recorded Investment [Line Items]
2020	75,329
2019	255,940
2018	228,293
2017	165,782
2016	80,025
Prior	38,299
Revolving Loans Amortized Cost Basis	0
Revolving Loans Converted to Term	0
Total	843,668
Consumer Indirect [Member] \| Performing [Member]
Financing Receivable Recorded Investment [Line Items]
2020	75,304
2019	255,499
2018	227,717
2017	165,416
2016	79,736
Prior	38,219
Revolving Loans Amortized Cost Basis	0
Revolving Loans Converted to Term	0
Total	841,891
Consumer Indirect [Member] \| Non-Performing [Member]
Financing Receivable Recorded Investment [Line Items]
2020	25
2019	441
2018	576
2017	366
2016	289
Prior	80
Revolving Loans Amortized Cost Basis	0
Revolving Loans Converted to Term	0
Total	1,777
Other Consumer [Member]
Financing Receivable Recorded Investment [Line Items]
2020	1,646
2019	4,889
2018	2,868
2017	1,672
2016	730
Prior	690
Revolving Loans Amortized Cost Basis	2,907
Revolving Loans Converted to Term	0
Total	15,402
Other Consumer [Member] \| Performing [Member]
Financing Receivable Recorded Investment [Line Items]
2020	1,646
2019	4,889
2018	2,868
2017	1,672
2016	730
Prior	690
Revolving Loans Amortized Cost Basis	2,907
Revolving Loans Converted to Term	0
Total	15,402
Other Consumer [Member] \| Non-Performing [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 revolving loans converted to term.

No definition available.

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

- Definition

Amount, after fee received for commitment to originate or purchase, of financing receivable. Excludes financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919230-210447

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5074-111524

Reference 3: 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=118955255&loc=SL82922352-210448

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI http://asc.fasb.org/extlink&oid=118955255&loc=SL82922355-210448

+ Details

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

- Definition

Amount of financing receivable originated five or more years prior to latest 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=118955255&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=119407570&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 latest 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 latest 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=119407570&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=118955255&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 latest 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 latest 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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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