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Premises And Equipment, Net (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Premises and equipment	$ 77,476us-gaap_PropertyPlantAndEquipmentGross	$ 75,177us-gaap_PropertyPlantAndEquipmentGross
Accumulated depreciation and amortization	(41,082)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment	(39,168)us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment
Premises and equipment, net	36,394us-gaap_PropertyPlantAndEquipmentNet	36,009us-gaap_PropertyPlantAndEquipmentNet
Depreciation and amortization	4,583us-gaap_DepreciationAndAmortization	4,181us-gaap_DepreciationAndAmortization	3,828us-gaap_DepreciationAndAmortization
Land And Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment	4,797us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandAndLandImprovementsMember	4,883us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_LandAndLandImprovementsMember
Buildings And Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment	42,826us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = fisi_BuildingsAndLeaseholdImprovementsMember	44,830us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = fisi_BuildingsAndLeaseholdImprovementsMember
Furniture Fixtures Equipment And Vehicles [Member]
Property, Plant and Equipment [Line Items]
Premises and equipment	29,853us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = fisi_FurnitureFixturesEquipmentAndVehiclesMember	25,464us-gaap_PropertyPlantAndEquipmentGross / us-gaap_PropertyPlantAndEquipmentByTypeAxis = fisi_FurnitureFixturesEquipmentAndVehiclesMember
Occupancy And Equipment Expense [Member]
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 4,000us-gaap_DepreciationAndAmortization / us-gaap_IncomeStatementLocationAxis = fisi_OccupancyAndEquipmentExpenseMember	$ 3,800us-gaap_DepreciationAndAmortization / us-gaap_IncomeStatementLocationAxis = fisi_OccupancyAndEquipmentExpenseMember	$ 3,600us-gaap_DepreciationAndAmortization / us-gaap_IncomeStatementLocationAxis = fisi_OccupancyAndEquipmentExpenseMember

X

- Definition

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.14)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 14

-Article 5

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- Definition

The current period expense charged against earnings on long-lived, physical assets not used in production, and which are not intended for resale, to allocate or recognize the cost of such assets over their useful lives; or to record the reduction in book value of an intangible asset over the benefit period of such asset; or to reflect consumption during the period of an asset that is not used in production.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229



Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

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Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

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-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229



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- Definition

Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229



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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

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-Section S99

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-Subparagraph (SX 210.5-02.13)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



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