Loans (Changes In The Allowance For Loan Losses) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, Ending balance	$ 3,110,064	$ 2,858,843	$ 3,110,064	$ 2,858,843	$ 3,044,320
Loans, Individually Evaluated for impairment	8,042	6,792	8,042	6,792
Loans, Collectively Evaluated for impairment	3,102,022	2,852,051	3,102,022	2,852,051
Ending balance	34,434	33,955	34,434	33,955
Allowance for loan losses, Individually Evaluated for impairment	3,107	1,743	3,107	1,743
Allowance for loan losses, Collectively Evaluated for impairment	31,327	32,212	31,327	32,212
Beginning balance	33,327	35,594	33,914	34,672
Charge-offs	(3,173)	(3,209)	(6,625)	(6,858)
Recoveries	1,926	1,530	3,598	3,152
Provision (credit)	2,354	40	3,547	2,989
Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, Ending balance	594,121	506,265	594,121	506,265	557,040
Loans, Individually Evaluated for impairment	768	4,164	768	4,164
Loans, Collectively Evaluated for impairment	593,353	502,101	593,353	502,101
Ending balance	11,717	14,242	11,717	14,242
Allowance for loan losses, Individually Evaluated for impairment	110	1,226	110	1,226
Allowance for loan losses, Collectively Evaluated for impairment	11,607	13,016	11,607	13,016
Beginning balance	12,167	14,942	14,312	15,668
Charge-offs	(138)	(336)	(268)	(441)
Recoveries	128	77	231	197
Provision (credit)	(440)	(441)	(2,558)	(1,182)
Commercial Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, Ending balance	1,011,925	869,192	1,011,925	869,192	960,265
Loans, Individually Evaluated for impairment	7,274	2,628	7,274	2,628
Loans, Collectively Evaluated for impairment	1,004,651	866,564	1,004,651	866,564
Ending balance	8,790	5,371	8,790	5,371
Allowance for loan losses, Individually Evaluated for impairment	2,997	517	2,997	517
Allowance for loan losses, Collectively Evaluated for impairment	5,793	4,854	5,793	4,854
Beginning balance	6,316	5,473	5,219	3,696
Charge-offs	(3)		(3)	(4)
Recoveries		1	17	8
Provision (credit)	2,477	(103)	3,557	1,671
Residential Real Estate Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, Ending balance	535,873	481,841	535,873	481,841	514,981
Loans, Collectively Evaluated for impairment	535,873	481,841	535,873	481,841
Ending balance	1,217	1,255	1,217	1,255
Allowance for loan losses, Collectively Evaluated for impairment	1,217	1,255	1,217	1,255
Beginning balance	1,265	1,400	1,112	1,322
Charge-offs	(87)	(10)	(118)	(29)
Recoveries	11	63	17	132
Provision (credit)	28	(198)	206	(170)
Residential Real Estate Lines [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, Ending balance	104,937	110,319	104,937	110,319	106,712
Loans, Collectively Evaluated for impairment	104,937	110,319	104,937	110,319
Ending balance	145	248	145	248
Allowance for loan losses, Collectively Evaluated for impairment	145	248	145	248
Beginning balance	173	218	210	180
Charge-offs	(2)	(7)	(2)	(101)
Recoveries	3	12	5	15
Provision (credit)	(29)	25	(68)	154
Consumer Indirect [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, Ending balance	846,829	874,718	846,829	874,718	888,732
Loans, Collectively Evaluated for impairment	846,829	874,718	846,829	874,718
Ending balance	12,157	12,520	12,157	12,520
Allowance for loan losses, Collectively Evaluated for impairment	12,157	12,520	12,157	12,520
Beginning balance	13,025	13,232	12,572	13,415
Charge-offs	(2,700)	(2,621)	(5,682)	(5,615)
Recoveries	1,678	1,304	3,102	2,634
Provision (credit)	154	605	2,165	2,086
Other Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans, Ending balance	16,379	16,508	16,379	16,508	$ 16,590
Loans, Collectively Evaluated for impairment	16,379	16,508	16,379	16,508
Ending balance	408	319	408	319
Allowance for loan losses, Collectively Evaluated for impairment	408	319	408	319
Beginning balance	381	329	489	391
Charge-offs	(243)	(235)	(552)	(668)
Recoveries	106	73	226	166
Provision (credit)	$ 164	$ 152	$ 245	$ 430

A valuation allowance for financing receivables that are expected to be uncollectible.

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

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_FinancingReceivableAllowanceForCreditLossesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of recovery of financing receivables doubtful of collection that were previously charged off.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of direct write-downs of financing receivables charged against the allowance.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

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

- Definition

Amount representing an agreement for an unconditional promise by the maker to pay the entity (holder) a definite sum of money at a future date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. Excludes amounts related to receivables held-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(3))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

Name:	us-gaap_ProvisionForLoanLeaseAndOtherLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fisi_CommercialAndIndustrialMember
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:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
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: