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Income Taxes (Income Tax Expense Differed From Statutory Federal Income Tax Rate) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes [Abstract]
Statutory federal tax rate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Tax exempt interest income	(5.10%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxExemptIncome	(4.80%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxExemptIncome	(4.60%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxExemptIncome
Tax credits and adjustments	(3.50%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsOther
Non-taxable earnings on company owned life insurance	(1.60%)fisi_EffectiveIncomeTaxRateReconciliationNonTaxableEarningsOnCompanyOwnedLifeInsurance	(1.60%)fisi_EffectiveIncomeTaxRateReconciliationNonTaxableEarningsOnCompanyOwnedLifeInsurance	(1.80%)fisi_EffectiveIncomeTaxRateReconciliationNonTaxableEarningsOnCompanyOwnedLifeInsurance
State taxes, net of federal tax benefit	(0.80%)us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	3.80%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	3.80%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Nondeductible expenses	0.50%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpense	0.20%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpense	0.30%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpense
Other, net	0.20%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments	0.10%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments	(0.10%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Effective tax rate	24.70%us-gaap_EffectiveIncomeTaxRateContinuingOperations	32.70%us-gaap_EffectiveIncomeTaxRateContinuingOperations	32.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations

X

- Definition

Effective Income Tax Rate Reconciliation Non Taxable Earnings On Company Owned Life Insurance

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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-Publisher FASB

-Name Accounting Standards Codification

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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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 attributable to nondeductible expenses.

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