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Income Taxes (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
State income tax rate	(0.80%)us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	3.80%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	3.80%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Domestic Country [Member]
Income Tax Contingency [Line Items]
Operating Loss Carryforwards	0us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember
State And Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Operating Loss Carryforwards	0us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember
Tax Credit Carryforward, Amount	900us-gaap_TaxCreditCarryforwardAmount / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember
New York [Member]
Income Tax Contingency [Line Items]
Taxable income apportioned to state	100.00%fisi_TaxableIncomeApportionedToState / us-gaap_IncomeTaxAuthorityAxis = stpr_NY
State income tax rate	7.10%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes / us-gaap_IncomeTaxAuthorityAxis = stpr_NY
New York [Member] | Scenario, Forecast [Member]
Income Tax Contingency [Line Items]
Taxable income apportioned to state					85.00%fisi_TaxableIncomeApportionedToState / us-gaap_IncomeTaxAuthorityAxis = stpr_NY / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
State income tax rate				6.50%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes / us-gaap_IncomeTaxAuthorityAxis = stpr_NY / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember
Minimum [Member] | Domestic Country [Member]
Income Tax Contingency [Line Items]
Federal income tax years currently open for audtis	2011
Minimum [Member] | State And Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Federal income tax years currently open for audtis	2011
Maximum [Member] | Domestic Country [Member]
Income Tax Contingency [Line Items]
Federal income tax years currently open for audtis	2014
Maximum [Member] | State And Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Federal income tax years currently open for audtis	2014

X

- Definition

Taxable Income Apportioned to State

+ References

No definition available.

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- Definition

Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319



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-Name Accounting Standards Codification

-Topic 740

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-Section S99

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-Name Accounting Standards Codification

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- Definition

Tax year that remains open to examination under enacted tax laws, in CCYY format.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

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-Paragraph 15

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- Definition

Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

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- Definition

The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 3

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-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319



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-URI http://asc.fasb.org/extlink&oid=6506874

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