<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>541</ContextCount>
  <ElementCount>498</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>115</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>11</UnitCount>
  <MyReports>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>100000 - Document - Document And Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document And Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>100010 - Statement - Consolidated Statements of Financial Condition (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/StatementConsolidatedStatementsOfFinancialConditionUnaudited</Role>
      <ShortName>Consolidated Statements of Financial Condition (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>100020 - Statement - Consolidated Statements of Financial Condition (Unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/StatementConsolidatedStatementsOfFinancialConditionUnauditedParenthetical</Role>
      <ShortName>Consolidated Statements of Financial Condition (Unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>100030 - Statement - Consolidated Statements of Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/StatementConsolidatedStatementsOfIncomeUnaudited</Role>
      <ShortName>Consolidated Statements of Income (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>100040 - Statement - Consolidated Statements of Comprehensive Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/StatementConsolidatedStatementsOfComprehensiveIncomeUnaudited</Role>
      <ShortName>Consolidated Statements of Comprehensive Income (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>100050 - Statement - Consolidated Statements of Changes in Shareholders' Equity (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/StatementConsolidatedStatementsOfChangesInShareholdersEquityUnaudited</Role>
      <ShortName>Consolidated Statements of Changes in Shareholders' Equity (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>100060 - Statement - Consolidated Statements of Changes in Shareholders' Equity (Unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/StatementConsolidatedStatementsOfChangesInShareholdersEquityUnauditedParenthetical</Role>
      <ShortName>Consolidated Statements of Changes in Shareholders' Equity (Unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>100070 - Statement - Consolidated Statements of Cash Flows (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/StatementConsolidatedStatementsOfCashFlowsUnaudited</Role>
      <ShortName>Consolidated Statements of Cash Flows (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>100080 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>100090 - Disclosure - Earnings Per Common Share ("EPS")</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEarningsPerCommonShareEPS</Role>
      <ShortName>Earnings Per Common Share ("EPS")</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>100100 - Disclosure - Investment Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureInvestmentSecurities</Role>
      <ShortName>Investment Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>100110 - Disclosure - Loans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoans</Role>
      <ShortName>Loans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>100120 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLeases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>100130 - Disclosure - Goodwill and Other Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureGoodwillAndOtherIntangibleAssets</Role>
      <ShortName>Goodwill and Other Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>100140 - Disclosure - Derivative Instruments and Hedging Activities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureDerivativeInstrumentsAndHedgingActivities</Role>
      <ShortName>Derivative Instruments and Hedging Activities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>100150 - Disclosure - Shareholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareholdersEquity</Role>
      <ShortName>Shareholders' Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>100160 - Disclosure - Accumulated Other Comprehensive Income (Loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncomeLoss</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>100170 - Disclosure - Share-Based Compensation Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareBasedCompensationPlans</Role>
      <ShortName>Share-Based Compensation Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>100180 - Disclosure - Employee Benefit Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEmployeeBenefitPlans</Role>
      <ShortName>Employee Benefit Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>100190 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>100200 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureFairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>100210 - Disclosure - Segment Reporting</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureSegmentReporting</Role>
      <ShortName>Segment Reporting</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>100220 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>100230 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>100240 - Disclosure - Earnings Per Common Share ("EPS") (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEarningsPerCommonShareEPSTables</Role>
      <ShortName>Earnings Per Common Share ("EPS") (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEarningsPerCommonShareEPS</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>100250 - Disclosure - Investment Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureInvestmentSecuritiesTables</Role>
      <ShortName>Investment Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureInvestmentSecurities</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>100260 - Disclosure - Loans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansTables</Role>
      <ShortName>Loans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoans</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>100270 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLeases</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>100280 - Disclosure - Goodwill and Other Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureGoodwillAndOtherIntangibleAssetsTables</Role>
      <ShortName>Goodwill and Other Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureGoodwillAndOtherIntangibleAssets</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>100290 - Disclosure - Derivative Instruments and Hedging Activities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureDerivativeInstrumentsAndHedgingActivitiesTables</Role>
      <ShortName>Derivative Instruments and Hedging Activities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureDerivativeInstrumentsAndHedgingActivities</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>100300 - Disclosure - Shareholders' Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareholdersEquityTables</Role>
      <ShortName>Shareholders' Equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareholdersEquity</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>100310 - Disclosure - Accumulated Other Comprehensive Income (Loss) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncomeLossTables</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncomeLoss</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>100320 - Disclosure - Share-Based Compensation Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareBasedCompensationPlansTables</Role>
      <ShortName>Share-Based Compensation Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareBasedCompensationPlans</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>100330 - Disclosure - Employee Benefit Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEmployeeBenefitPlansTables</Role>
      <ShortName>Employee Benefit Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEmployeeBenefitPlans</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>100340 - Disclosure - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureCommitmentsAndContingenciesTables</Role>
      <ShortName>Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureCommitmentsAndContingencies</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>100350 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureFairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureFairValueMeasurements</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>100360 - Disclosure - Segment Reporting (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureSegmentReportingTables</Role>
      <ShortName>Segment Reporting (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureSegmentReporting</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>100370 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesNarrativeDetails</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesTables</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>100380 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies (Summary of Supplemental Cash Flow Information) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesSummaryOfSupplementalCashFlowInformationDetails</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies (Summary of Supplemental Cash Flow Information) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesTables</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>100390 - Disclosure - Earnings Per Common Share ("EPS") (Reconciliation of Earnings and Shares Used in Calculating Basic and Diluted EPS) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEarningsPerCommonShareEPSReconciliationOfEarningsAndSharesUsedInCalculatingBasicAndDilutedEPSDetails</Role>
      <ShortName>Earnings Per Common Share ("EPS") (Reconciliation of Earnings and Shares Used in Calculating Basic and Diluted EPS) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEarningsPerCommonShareEPSTables</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>100400 - Disclosure - Earnings Per Common Share ("EPS") (Shares Excluded from Computation of Diluted EPS) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEarningsPerCommonShareEPSSharesExcludedFromComputationOfDilutedEPSDetails</Role>
      <ShortName>Earnings Per Common Share ("EPS") (Shares Excluded from Computation of Diluted EPS) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEarningsPerCommonShareEPSTables</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>100410 - Disclosure - Investment Securities (Amortized Cost and Fair Value of Investment Securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureInvestmentSecuritiesAmortizedCostAndFairValueOfInvestmentSecuritiesDetails</Role>
      <ShortName>Investment Securities (Amortized Cost and Fair Value of Investment Securities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureInvestmentSecuritiesTables</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>100420 - Disclosure - Investment Securities (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureInvestmentSecuritiesNarrativeDetails</Role>
      <ShortName>Investment Securities (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureInvestmentSecuritiesTables</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>100430 - Disclosure - Investment Securities (Sales of Securities Available for Sale) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureInvestmentSecuritiesSalesOfSecuritiesAvailableForSaleDetails</Role>
      <ShortName>Investment Securities (Sales of Securities Available for Sale) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureInvestmentSecuritiesTables</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>100440 - Disclosure - Investment Securities (Scheduled Maturities of Securities Available for Sale and Securities Held to Maturity) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureInvestmentSecuritiesScheduledMaturitiesOfSecuritiesAvailableForSaleAndSecuritiesHeldToMaturityDetails</Role>
      <ShortName>Investment Securities (Scheduled Maturities of Securities Available for Sale and Securities Held to Maturity) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureInvestmentSecuritiesTables</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>100450 - Disclosure - Investment Securities (Investments Gross Unrealized Losses and Fair Value) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureInvestmentSecuritiesInvestmentsGrossUnrealizedLossesAndFairValueDetails</Role>
      <ShortName>Investment Securities (Investments Gross Unrealized Losses and Fair Value) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureInvestmentSecuritiesTables</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>100460 - Disclosure - Loans (Loan Portfolio) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansLoanPortfolioDetails</Role>
      <ShortName>Loans (Loan Portfolio) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansTables</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>100470 - Disclosure - Loans (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansNarrativeDetails</Role>
      <ShortName>Loans (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansTables</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>100480 - Disclosure - Loans (Recorded Investment by Loan Class in Current and Nonaccrual Loans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansRecordedInvestmentByLoanClassInCurrentAndNonaccrualLoansDetails</Role>
      <ShortName>Loans (Recorded Investment by Loan Class in Current and Nonaccrual Loans) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansTables</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>100490 - Disclosure - Loans (Information Related to Loans Modified in TDR) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansInformationRelatedToLoansModifiedInTDRDetails</Role>
      <ShortName>Loans (Information Related to Loans Modified in TDR) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansTables</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>100500 - Disclosure - Loans (Summary of Collateral Dependent Loans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansSummaryOfCollateralDependentLoansDetails</Role>
      <ShortName>Loans (Summary of Collateral Dependent Loans) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansTables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>100510 - Disclosure - Loans (Commercial Loan Portfolio Categorized by Internally Assigned Asset Classification) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansCommercialLoanPortfolioCategorizedByInternallyAssignedAssetClassificationDetails</Role>
      <ShortName>Loans (Commercial Loan Portfolio Categorized by Internally Assigned Asset Classification) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansTables</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>100520 - Disclosure - Loans (Retail Loan Portfolio Categorized by Performance Status) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansRetailLoanPortfolioCategorizedByPerformanceStatusDetails</Role>
      <ShortName>Loans (Retail Loan Portfolio Categorized by Performance Status) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansTables</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>100530 - Disclosure - Loans (Changes in the Allowance for Credit Losses) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansChangesInAllowanceForCreditLossesDetails</Role>
      <ShortName>Loans (Changes in the Allowance for Credit Losses) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLoansTables</ParentRole>
      <Position>54</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>100540 - Disclosure - Leases (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLeasesNarrativeDetails</Role>
      <ShortName>Leases (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLeasesTables</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>100550 - Disclosure - Leases (Summary of Classification of Right of Use Assets and Lease Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLeasesSummaryOfClassificationOfRightOfUseAssetsAndLeaseLiabilitiesDetails</Role>
      <ShortName>Leases (Summary of Classification of Right of Use Assets and Lease Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLeasesTables</ParentRole>
      <Position>56</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>100560 - Disclosure - Leases (Summary of Lease Costs and Other Lease Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLeasesSummaryOfLeaseCostsAndOtherLeaseInformationDetails</Role>
      <ShortName>Leases (Summary of Lease Costs and Other Lease Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLeasesTables</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>100570 - Disclosure - Leases (Summary of Future Minimum Payments Under Non-cancellable Operating Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLeasesSummaryOfFutureMinimumPaymentsUnderNonCancellableOperatingLeasesDetails</Role>
      <ShortName>Leases (Summary of Future Minimum Payments Under Non-cancellable Operating Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureLeasesTables</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>100590 - Disclosure - Goodwill and Other Intangible Assets (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureGoodwillAndOtherIntangibleAssetsNarrativeDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureGoodwillAndOtherIntangibleAssetsTables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>100600 - Disclosure - Goodwill and Other Intangible Assets (Changes in Carrying Amount of Goodwill) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureGoodwillAndOtherIntangibleAssetsChangesInCarryingAmountOfGoodwillDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets (Changes in Carrying Amount of Goodwill) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureGoodwillAndOtherIntangibleAssetsTables</ParentRole>
      <Position>60</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>100610 - Disclosure - Goodwill and Other Intangible Assets (Gross Carrying Amount Accumulated Amortization and Net Book Value) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureGoodwillAndOtherIntangibleAssetsGrossCarryingAmountAccumulatedAmortizationAndNetBookValueDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets (Gross Carrying Amount Accumulated Amortization and Net Book Value) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureGoodwillAndOtherIntangibleAssetsTables</ParentRole>
      <Position>61</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>100620 - Disclosure - Goodwill and Other Intangible Assets (Estimated Amortization Expense of Other Intangible Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureGoodwillAndOtherIntangibleAssetsEstimatedAmortizationExpenseOfOtherIntangibleAssetsDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets (Estimated Amortization Expense of Other Intangible Assets) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureGoodwillAndOtherIntangibleAssetsTables</ParentRole>
      <Position>62</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>100630 - Disclosure - Derivative Instruments and Hedging Activities (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureDerivativeInstrumentsAndHedgingActivitiesNarrativeDetails</Role>
      <ShortName>Derivative Instruments and Hedging Activities (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureDerivativeInstrumentsAndHedgingActivitiesTables</ParentRole>
      <Position>63</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>100640 - Disclosure - Derivative Instruments and Hedging Activities (Fair Values of Derivative Instruments on the Balance Sheet) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureDerivativeInstrumentsAndHedgingActivitiesFairValuesOfDerivativeInstrumentsOnBalanceSheetDetails</Role>
      <ShortName>Derivative Instruments and Hedging Activities (Fair Values of Derivative Instruments on the Balance Sheet) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureDerivativeInstrumentsAndHedgingActivitiesTables</ParentRole>
      <Position>64</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>100650 - Disclosure - Derivative Instruments and Hedging Activities (Fair Values of Derivative Instruments on the Balance Sheet) (Parenthetical) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureDerivativeInstrumentsAndHedgingActivitiesFairValuesOfDerivativeInstrumentsOnBalanceSheetParentheticalDetails</Role>
      <ShortName>Derivative Instruments and Hedging Activities (Fair Values of Derivative Instruments on the Balance Sheet) (Parenthetical) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureDerivativeInstrumentsAndHedgingActivitiesTables</ParentRole>
      <Position>65</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>100660 - Disclosure - Derivative Instruments and Hedging Activities (Effect of Derivative Instruments on the Income Statement) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureDerivativeInstrumentsAndHedgingActivitiesEffectOfDerivativeInstrumentsOnIncomeStatementDetails</Role>
      <ShortName>Derivative Instruments and Hedging Activities (Effect of Derivative Instruments on the Income Statement) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureDerivativeInstrumentsAndHedgingActivitiesTables</ParentRole>
      <Position>66</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>100670 - Disclosure - Shareholders' Equity (Changes in Shares of Common Stock) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareholdersEquityChangesInSharesOfCommonStockDetails</Role>
      <ShortName>Shareholders' Equity (Changes in Shares of Common Stock) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareholdersEquityTables</ParentRole>
      <Position>67</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>100680 - Disclosure - Accumulated Other Comprehensive Income (Loss) (Components of Other Comprehensive Income (Loss)) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncomeLossComponentsOfOtherComprehensiveIncomeLossDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss) (Components of Other Comprehensive Income (Loss)) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncomeLossTables</ParentRole>
      <Position>68</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>100690 - Disclosure - Accumulated Other Comprehensive Income (Loss) (Components of Accumulated Other Comprehensive Income (Loss)) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncomeLossComponentsOfAccumulatedOtherComprehensiveIncomeLossDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss) (Components of Accumulated Other Comprehensive Income (Loss)) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncomeLossTables</ParentRole>
      <Position>69</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>100700 - Disclosure - Accumulated Other Comprehensive Income (Loss) (Amounts Reclassified Out of Each Component Of Accumulated Other Comprehensive Income (Loss)) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncomeLossAmountsReclassifiedOutOfEachComponentOfAccumulatedOtherComprehensiveIncomeLossDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss) (Amounts Reclassified Out of Each Component Of Accumulated Other Comprehensive Income (Loss)) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureAccumulatedOtherComprehensiveIncomeLossTables</ParentRole>
      <Position>70</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>100710 - Disclosure - Share-Based Compensation Plans (Summary Of Restricted Stock Units And Performance Share Units Activity) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareBasedCompensationPlansSummaryOfRestrictedStockUnitsAndPerformanceShareUnitsActivityDetails</Role>
      <ShortName>Share-Based Compensation Plans (Summary Of Restricted Stock Units And Performance Share Units Activity) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareBasedCompensationPlansTables</ParentRole>
      <Position>71</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>100720 - Disclosure - Share-Based Compensation Plans (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareBasedCompensationPlansNarrativeDetails</Role>
      <ShortName>Share-Based Compensation Plans (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareBasedCompensationPlansTables</ParentRole>
      <Position>72</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>100730 - Disclosure - Share-Based Compensation Plans (Summary of Restricted Stock Awards and Restricted Stock Units Activity) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareBasedCompensationPlansSummaryOfRestrictedStockAwardsAndRestrictedStockUnitsActivityDetails</Role>
      <ShortName>Share-Based Compensation Plans (Summary of Restricted Stock Awards and Restricted Stock Units Activity) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareBasedCompensationPlansTables</ParentRole>
      <Position>73</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>100740 - Disclosure - Share-Based Compensation Plans (Share-Based Compensation Expense Included In Consolidated Statements Of Income) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareBasedCompensationPlansShareBasedCompensationExpenseIncludedInConsolidatedStatementsOfIncomeDetails</Role>
      <ShortName>Share-Based Compensation Plans (Share-Based Compensation Expense Included In Consolidated Statements Of Income) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureShareBasedCompensationPlansTables</ParentRole>
      <Position>74</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>100750 - Disclosure - Employee Benefit Plans (Components of Net Periodic Benefit Expense) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEmployeeBenefitPlansComponentsOfNetPeriodicBenefitExpenseDetails</Role>
      <ShortName>Employee Benefit Plans (Components of Net Periodic Benefit Expense) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEmployeeBenefitPlansTables</ParentRole>
      <Position>75</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>100760 - Disclosure - Employee Benefit Plans (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEmployeeBenefitPlansNarrativeDetails</Role>
      <ShortName>Employee Benefit Plans (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureEmployeeBenefitPlansTables</ParentRole>
      <Position>76</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>100770 - Disclosure - Commitments and Contingencies (Off-Balance Sheet Commitments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureCommitmentsAndContingenciesOffBalanceSheetCommitmentsDetails</Role>
      <ShortName>Commitments and Contingencies (Off-Balance Sheet Commitments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureCommitmentsAndContingenciesTables</ParentRole>
      <Position>77</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>100780 - Disclosure - Commitments and Contingencies (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureCommitmentsAndContingenciesNarrativeDetails</Role>
      <ShortName>Commitments and Contingencies (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureCommitmentsAndContingenciesTables</ParentRole>
      <Position>78</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>100790 - Disclosure - Fair Value Measurements (Assets Measured at Fair Value on a Recurring and Non-Recurring Basis) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureFairValueMeasurementsAssetsMeasuredAtFairValueOnRecurringAndNonRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements (Assets Measured at Fair Value on a Recurring and Non-Recurring Basis) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureFairValueMeasurementsTables</ParentRole>
      <Position>79</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>100800 - Disclosure - Fair Value Measurements (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureFairValueMeasurementsNarrativeDetails</Role>
      <ShortName>Fair Value Measurements (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureFairValueMeasurementsTables</ParentRole>
      <Position>80</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>100810 - Disclosure - Fair Value Measurements (Additional Quantitative Information about Assets Measured at Fair Value on a Recurring and Non-Recurring Basis) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureFairValueMeasurementsAdditionalQuantitativeInformationAboutAssetsMeasuredAtFairValueOnRecurringAndNonRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements (Additional Quantitative Information about Assets Measured at Fair Value on a Recurring and Non-Recurring Basis) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureFairValueMeasurementsTables</ParentRole>
      <Position>81</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>100820 - Disclosure - Fair Value Measurements (Carrying Amount, Estimated Fair Value, and Placement in Fair Value Hierarchy of Financial Instruments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureFairValueMeasurementsCarryingAmountEstimatedFairValueAndPlacementInFairValueHierarchyOfFinancialInstrumentsDetails</Role>
      <ShortName>Fair Value Measurements (Carrying Amount, Estimated Fair Value, and Placement in Fair Value Hierarchy of Financial Instruments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureFairValueMeasurementsTables</ParentRole>
      <Position>82</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>100830 - Disclosure - Segment Reporting (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureSegmentReportingNarrativeDetails</Role>
      <ShortName>Segment Reporting (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureSegmentReportingTables</ParentRole>
      <Position>83</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>100840 - Disclosure - Segment Reporting (Business Segment Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureSegmentReportingBusinessSegmentAssetsDetails</Role>
      <ShortName>Segment Reporting (Business Segment Assets) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureSegmentReportingTables</ParentRole>
      <Position>84</Position>
    </Report>
    <Report instance="fisi-10q_20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>100850 - Disclosure - Segment Reporting (Business Segment Profit (Loss)) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureSegmentReportingBusinessSegmentProfitLossDetails</Role>
      <ShortName>Segment Reporting (Business Segment Profit (Loss)) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fiiwarsaw.com/20200630/taxonomy/role/DisclosureSegmentReportingTables</ParentRole>
      <Position>85</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" original="fisi-10q_20200630.htm">fisi-10q_20200630.htm</File>
    <File>fisi-20200630.xsd</File>
    <File>fisi-20200630_cal.xml</File>
    <File>fisi-20200630_def.xml</File>
    <File>fisi-20200630_lab.xml</File>
    <File>fisi-20200630_pre.xml</File>
    <File>fisi-ex101_123.htm</File>
    <File>fisi-ex311_6.htm</File>
    <File>fisi-ex312_8.htm</File>
    <File>fisi-ex32_7.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/srt/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
