XML 126 R113.htm IDEA: XBRL DOCUMENT v3.8.0.1

Employee Benefit Plans (Changes In Fair Value Of Plan Assets) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of period	$ 75,252	$ 72,358
Fair value of plan assets at end of period	83,348	75,252
Level 3 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of period	2,637	2,670
Realized Gain	43	52
Purchases	103
Sales	(224)	(381)
Unrealized gains	82	296
Fair value of plan assets at end of period	$ 2,641	$ 2,637

X

- Definition

Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Purchases

+ References

No definition available.

+ Details

Name:	fisi_DefinedBenefitPlanPlanAssetsLevel3ReconciliationIncreaseDecreaseForPurchases
Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Sales

+ References

No definition available.

+ Details

Name:	fisi_DefinedBenefitPlanPlanAssetsLevel3ReconciliationIncreaseDecreaseForSales
Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Defined Benefit Plan, Realized Gain (Loss)

+ References

No definition available.

+ Details

Name:	fisi_DefinedBenefitPlanRealizedGainLoss
Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Defined Benefit Plan, Unrealized Gain (Loss)

+ References

No definition available.

+ Details

Name:	fisi_DefinedBenefitPlanUnrealizedGainLoss
Namespace Prefix:	fisi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- 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_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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

X

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: