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Income Taxes (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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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 | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember |  |  |  |
State And Local Jurisdiction [Member] |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember |  |  |  |
Tax Credit Carryforward, Amount | 900 us-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 |  |  |  |

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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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 + Details
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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 -Section 50 -Paragraph 15 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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- Definition Amount of operating loss 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 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319 + Details
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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 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Carryforwards -URI http://asc.fasb.org/extlink&oid=6506874 + Details
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