Investment Securities (Investments Gross Unrealized Losses and Fair Value) (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Less than 12 months, Fair Value	$ 8,644	$ 193,800
Securities available for sale, Less than 12 months, Unrealized Losses	661	17,922
Securities available for sale, 12 months or longer, Fair Value	903,146	760,210
Securities available for sale, 12 months or longer, Unrealized Losses	169,723	155,102
Securities available for sale, Fair Value, Total	911,790	954,010
Securities available for sale, Unrealized Losses, Total	170,384	173,024
Total AFS debt securities with unrealized losses, Less than 12 months, Fair Value	8,644	193,800
Total AFS debt securities with unrealized losses, Less than 12 months, Unrealized Losses	661	17,922
Total AFS debt securities with unrealized losses, 12 months or longer, Fair Value	903,146	760,210
Total AFS debt securities with unrealized losses, 12 months or longer, Unrealized Losses	169,723	155,102
Total AFS debt securities with unrealized losses, Fair Value	911,790	954,010
Total AFS debt securities with unrealized losses, Unrealized Losses	170,384	173,024
U.S. Government Agency And Government Sponsored Enterprises [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Less than 12 months, Fair Value	0	0
Securities available for sale, Less than 12 months, Unrealized Losses	0	0
Securities available for sale, 12 months or longer, Fair Value	21,261	21,115
Securities available for sale, 12 months or longer, Unrealized Losses	3,274	3,420
Securities available for sale, Fair Value, Total	21,261	21,115
Securities available for sale, Unrealized Losses, Total	3,274	3,420
Collateralized Mortgage Obligations [Member] \| Federal National Mortgage Association [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Less than 12 months, Fair Value	0	1,000
Securities available for sale, Less than 12 months, Unrealized Losses	0	94
Securities available for sale, 12 months or longer, Fair Value	8,958	8,572
Securities available for sale, 12 months or longer, Unrealized Losses	2,544	2,509
Securities available for sale, Fair Value, Total	8,958	9,572
Securities available for sale, Unrealized Losses, Total	2,544	2,603
Collateralized Mortgage Obligations [Member] \| Federal Home Loan Mortgage Corporation [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Less than 12 months, Fair Value	0	0
Securities available for sale, Less than 12 months, Unrealized Losses	0	0
Securities available for sale, 12 months or longer, Fair Value	16,089	17,356
Securities available for sale, 12 months or longer, Unrealized Losses	4,584	4,163
Securities available for sale, Fair Value, Total	16,089	17,356
Securities available for sale, Unrealized Losses, Total	4,584	4,163
Mortgage-Backed Securities [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Less than 12 months, Fair Value	8,644	193,800
Securities available for sale, Less than 12 months, Unrealized Losses	661	17,922
Securities available for sale, 12 months or longer, Fair Value	881,885	739,095
Securities available for sale, 12 months or longer, Unrealized Losses	166,449	151,682
Securities available for sale, Fair Value, Total	890,529	932,895
Securities available for sale, Unrealized Losses, Total	167,110	169,604
Mortgage-Backed Securities [Member] \| Federal National Mortgage Association [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Less than 12 months, Fair Value	183	154,006
Securities available for sale, Less than 12 months, Unrealized Losses	4	14,708
Securities available for sale, 12 months or longer, Fair Value	451,466	315,598
Securities available for sale, 12 months or longer, Unrealized Losses	73,144	61,485
Securities available for sale, Fair Value, Total	451,649	469,604
Securities available for sale, Unrealized Losses, Total	73,148	76,193
Mortgage-Backed Securities [Member] \| Federal Home Loan Mortgage Corporation [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Less than 12 months, Fair Value	8,376	28,493
Securities available for sale, Less than 12 months, Unrealized Losses	654	2,199
Securities available for sale, 12 months or longer, Fair Value	316,317	313,728
Securities available for sale, 12 months or longer, Unrealized Losses	65,001	66,409
Securities available for sale, Fair Value, Total	324,693	342,221
Securities available for sale, Unrealized Losses, Total	65,655	68,608
Mortgage-Backed Securities [Member] \| Government National Mortgage Association [Member]
Schedule of Debt Securities Available For Sale and Held To Maturity Securities [Line Item]
Securities available for sale, Less than 12 months, Fair Value	85	10,301
Securities available for sale, Less than 12 months, Unrealized Losses	3	921
Securities available for sale, 12 months or longer, Fair Value	89,055	83,841
Securities available for sale, 12 months or longer, Unrealized Losses	21,176	17,116
Securities available for sale, Fair Value, Total	89,140	94,142
Securities available for sale, Unrealized Losses, Total	$ 21,179	$ 18,037

Debt securities available for sale and held to maturity securities.

No definition available.

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

- Definition

Amount of accumulated pre-tax unrealized loss on securities.

+ References

No definition available.

+ Details

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

- Definition

Amount of accumulated pre-tax unrealized loss on securities that have been in a loss position for twelve months or longer.

+ References

No definition available.

+ Details

Name:	fisi_InvestmentSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This item represents the fair value of securities which are in an unrealized loss position.

+ References

No definition available.

+ Details

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

- Definition

Amount of accumulated pre-tax unrealized loss on investments in securities that have been in a loss position for less than twelve months.

+ References

No definition available.

+ Details

Name:	fisi_InvestmentSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This item represents the fair value of securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

No definition available.

+ Details

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

- Definition

This item represents the fair value of securities that have been in a continuous unrealized loss position for twelve months or longer.

+ References

No definition available.

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479081/326-30-55-8

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-7

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479106/326-30-50-5

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479081/326-30-55-8

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-6

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479106/326-30-50-4

+ Details

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

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479106/326-30-50-4

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_FederalNationalMortgageAssociationCertificatesAndObligationsFNMAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: