Investment Securities (Investments Gross Unrealized Losses And Fair Value) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Securities available for sale, Less than 12 months, Fair Value	$ 70	$ 251
Securities available for sale, Less than 12 months, Unrealized Losses	1	1
Securities available for sale, 12 months or longer, Fair Value	373,183	438,290
Securities available for sale, 12 months or longer, Unrealized Losses	3,814	10,749
Securities available for sale, Fair Value, Total	373,253	438,541
Securities available for sale, Unrealized Losses, Total	3,815	10,750
Securities held to maturity, Less than 12 months, Fair Value	9,235	50,519
Securities held to maturity, Less than 12 months, Unrealized Losses	17	181
Securities held to maturity, 12 months or longer, Fair Value	201,487	234,507
Securities held to maturity, 12 months or longer, Unrealized Losses	4,412	7,718
Securities held to maturity, Fair Value, Total	210,722	285,026
Securities held to maturity, Unrealized Losses, Total	4,429	7,899
Total Securities, Less than 12 months, Fair Value	9,305	50,770
Total Securities, Less than 12 months, Unrealized Losses	18	182
Total Securities, 12 months or longer, Fair Value	574,670	672,797
Total Securities, 12 months or longer, Unrealized Losses	8,226	18,467
Total Securities, Fair Value	583,975	723,567
Total Securities, Unrealized Losses	8,244	18,649
U.S. Government Agency And Government Sponsored Enterprises [Member]
Investment [Line Items]
Securities available for sale, 12 months or longer, Fair Value	112,425	152,028
Securities available for sale, 12 months or longer, Unrealized Losses	809	3,074
Securities available for sale, Fair Value, Total	112,425	152,028
Securities available for sale, Unrealized Losses, Total	809	3,074
State And Political Subdivisions [Member]
Investment [Line Items]
Securities held to maturity, Less than 12 months, Fair Value	2,717	35,751
Securities held to maturity, Less than 12 months, Unrealized Losses	1	91
Securities held to maturity, 12 months or longer, Fair Value	24,312	49,534
Securities held to maturity, 12 months or longer, Unrealized Losses	224	1,120
Securities held to maturity, Fair Value, Total	27,029	85,285
Securities held to maturity, Unrealized Losses, Total	225	1,211
Collateralized Mortgage Obligations [Member] \| Federal National Mortgage Association [Member]
Investment [Line Items]
Securities available for sale, Less than 12 months, Fair Value	70
Securities available for sale, Less than 12 months, Unrealized Losses	1
Securities available for sale, 12 months or longer, Fair Value	19	56
Securities available for sale, Fair Value, Total	89	56
Securities available for sale, Unrealized Losses, Total	1
Securities held to maturity, 12 months or longer, Fair Value	56,114	57,973
Securities held to maturity, 12 months or longer, Unrealized Losses	1,474	2,179
Securities held to maturity, Fair Value, Total	56,114	57,973
Securities held to maturity, Unrealized Losses, Total	1,474	2,179
Collateralized Mortgage Obligations [Member] \| Federal Home Loan Mortgage Corporation [Member]
Investment [Line Items]
Securities available for sale, 12 months or longer, Fair Value	6	6
Securities available for sale, Fair Value, Total	6	6
Securities held to maturity, 12 months or longer, Fair Value	72,277	75,603
Securities held to maturity, 12 months or longer, Unrealized Losses	1,624	2,597
Securities held to maturity, Fair Value, Total	72,277	75,603
Securities held to maturity, Unrealized Losses, Total	1,624	2,597
Collateralized Mortgage Obligations [Member] \| Government National Mortgage Association [Member]
Investment [Line Items]
Securities held to maturity, 12 months or longer, Fair Value	16,561	17,263
Securities held to maturity, 12 months or longer, Unrealized Losses	363	535
Securities held to maturity, Fair Value, Total	16,561	17,263
Securities held to maturity, Unrealized Losses, Total	363	535
Mortgage-Backed Securities [Member]
Investment [Line Items]
Securities available for sale, Less than 12 months, Fair Value	70	251
Securities available for sale, Less than 12 months, Unrealized Losses	1	1
Securities available for sale, 12 months or longer, Fair Value	260,758	286,262
Securities available for sale, 12 months or longer, Unrealized Losses	3,005	7,675
Securities available for sale, Fair Value, Total	260,828	286,513
Securities available for sale, Unrealized Losses, Total	3,006	7,676
Securities held to maturity, Less than 12 months, Fair Value	6,518	14,768
Securities held to maturity, Less than 12 months, Unrealized Losses	16	90
Securities held to maturity, 12 months or longer, Fair Value	177,175	184,973
Securities held to maturity, 12 months or longer, Unrealized Losses	4,188	6,598
Securities held to maturity, Fair Value, Total	183,693	199,741
Securities held to maturity, Unrealized Losses, Total	4,204	6,688
Mortgage-Backed Securities [Member] \| Federal National Mortgage Association [Member]
Investment [Line Items]
Securities available for sale, Less than 12 months, Fair Value		251
Securities available for sale, Less than 12 months, Unrealized Losses		1
Securities available for sale, 12 months or longer, Fair Value	224,846	247,615
Securities available for sale, 12 months or longer, Unrealized Losses	2,366	6,324
Securities available for sale, Fair Value, Total	224,846	247,866
Securities available for sale, Unrealized Losses, Total	2,366	6,325
Securities held to maturity, Less than 12 months, Fair Value		1,518
Securities held to maturity, Less than 12 months, Unrealized Losses		3
Securities held to maturity, 12 months or longer, Fair Value	7,696	8,695
Securities held to maturity, 12 months or longer, Unrealized Losses	114	258
Securities held to maturity, Fair Value, Total	7,696	10,213
Securities held to maturity, Unrealized Losses, Total	114	261
Mortgage-Backed Securities [Member] \| Federal Home Loan Mortgage Corporation [Member]
Investment [Line Items]
Securities available for sale, 12 months or longer, Fair Value	31,365	33,918
Securities available for sale, 12 months or longer, Unrealized Losses	634	1,275
Securities available for sale, Fair Value, Total	31,365	33,918
Securities available for sale, Unrealized Losses, Total	634	1,275
Securities held to maturity, Less than 12 months, Fair Value		1,467
Securities held to maturity, Less than 12 months, Unrealized Losses		5
Securities held to maturity, 12 months or longer, Fair Value	2,619	2,923
Securities held to maturity, 12 months or longer, Unrealized Losses	109	188
Securities held to maturity, Fair Value, Total	2,619	4,390
Securities held to maturity, Unrealized Losses, Total	109	193
Mortgage-Backed Securities [Member] \| Government National Mortgage Association [Member]
Investment [Line Items]
Securities available for sale, 12 months or longer, Fair Value	4,522	4,667
Securities available for sale, 12 months or longer, Unrealized Losses	5	76
Securities available for sale, Fair Value, Total	4,522	4,667
Securities available for sale, Unrealized Losses, Total	5	76
Securities held to maturity, Less than 12 months, Fair Value	6,518	11,783
Securities held to maturity, Less than 12 months, Unrealized Losses	16	82
Securities held to maturity, 12 months or longer, Fair Value	21,908	22,516
Securities held to maturity, 12 months or longer, Unrealized Losses	504	841
Securities held to maturity, Fair Value, Total	28,426	34,299
Securities held to maturity, Unrealized Losses, Total	$ 520	$ 923

Amount of accumulated pre-tax unrealized loss on securities.

No definition available.

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

Line items represent reporting 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/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

+ Details

Name:	invest_InvestmentLineItems
Namespace Prefix:	invest_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

+ 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.

+ References

+ Details

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

- Definition

+ References

+ Details

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

- Definition

+ 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

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

- Definition

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
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 amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
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 amortized cost (held-to-maturity), in continuous loss position for 12 months or longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
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 amortized cost (held-to-maturity), in continuous loss position for less than 12 months.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for less than 12 months.

+ References

+ Details

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

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for 12 months or longer.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
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_USStatesAndPoliticalSubdivisionsMember
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_InvestmentTypeAxis=us-gaap_GovernmentNationalMortgageAssociationCertificatesAndObligationsGNMAMember
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: