Loans (Changes In The Allowance For Loan Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 30,934	$ 27,085	$ 27,637
Charge-offs	(15,251)	(11,076)	(12,958)
Recoveries	5,628	5,287	5,025
Provision (credit)	13,361	9,638	7,381
Ending balance	34,672	30,934	27,085
Allowance for loan losses, Individually Evaluated for impairment	2,108	768	1,006
Allowance for loan losses, Collectively Evaluated for impairment	32,564	30,166	26,079
Loans, Ending balance	2,694,922	2,305,071	2,052,600
Loans, Individually Evaluated for impairment	8,174	2,987	4,869
Loans, Collectively Evaluated for impairment	2,686,748	2,302,084	2,047,731
Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	7,225	5,540	5,621
Charge-offs	(3,614)	(943)	(1,433)
Recoveries	416	447	212
Provision (credit)	11,641	2,181	1,140
Ending balance	15,668	7,225	5,540
Allowance for loan losses, Individually Evaluated for impairment	2,001	663	996
Allowance for loan losses, Collectively Evaluated for impairment	13,667	6,562	4,544
Loans, Ending balance	449,763	349,079	313,475
Loans, Individually Evaluated for impairment	5,322	2,052	3,922
Loans, Collectively Evaluated for impairment	444,441	347,027	309,553
Commercial Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	10,315	9,027	8,122
Charge-offs	(10)	(385)	(895)
Recoveries	262	45	146
Provision (credit)	(6,871)	1,628	1,654
Ending balance	3,696	10,315	9,027
Allowance for loan losses, Individually Evaluated for impairment	107	105	10
Allowance for loan losses, Collectively Evaluated for impairment	3,589	10,210	9,017
Loans, Ending balance	810,851	671,552	567,481
Loans, Individually Evaluated for impairment	2,852	935	947
Loans, Collectively Evaluated for impairment	807,999	670,617	566,534
Residential Real Estate Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	1,478	1,347	1,620
Charge-offs	(431)	(289)	(397)
Recoveries	130	174	114
Provision (credit)	145	246	10
Ending balance	1,322	1,478	1,347
Allowance for loan losses, Collectively Evaluated for impairment	1,322	1,478	1,347
Loans, Ending balance	457,761	421,476	376,023
Loans, Collectively Evaluated for impairment	457,761	421,476	376,023
Residential Real Estate Lines [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	303	345	435
Charge-offs	(106)	(104)	(199)
Recoveries	60	15	31
Provision (credit)	(77)	47	78
Ending balance	180	303	345
Allowance for loan losses, Collectively Evaluated for impairment	180	303	345
Loans, Ending balance	113,422	119,745	124,766
Loans, Collectively Evaluated for impairment	113,422	119,745	124,766
Consumer Indirect [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	11,311	10,458	11,383
Charge-offs	(10,164)	(8,748)	(9,156)
Recoveries	4,444	4,259	4,200
Provision (credit)	7,824	5,342	4,031
Ending balance	13,415	11,311	10,458
Allowance for loan losses, Collectively Evaluated for impairment	13,415	11,311	10,458
Loans, Ending balance	845,682	725,754	652,494
Loans, Collectively Evaluated for impairment	845,682	725,754	652,494
Other Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	302	368	456
Charge-offs	(926)	(607)	(878)
Recoveries	316	347	322
Provision (credit)	699	194	468
Ending balance	391	302	368
Allowance for loan losses, Collectively Evaluated for impairment	391	302	368
Loans, Ending balance	17,443	17,465	18,361
Loans, Collectively Evaluated for impairment	$ 17,443	$ 17,465	$ 18,361

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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&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: