<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.6.0.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>640</ContextCount>
  <ElementCount>683</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>131</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>13</UnitCount>
  <MyReports>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000100 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/DocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>010000 - Statement - CONSOLIDATED STATEMENTS OF CONDITION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/ConsolidatedStatementsOfCondition</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CONDITION</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>010100 - Statement - CONSOLIDATED STATEMENTS OF CONDITION (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/ConsolidatedStatementsOfConditionParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CONDITION (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>020000 - Statement - CONSOLIDATED STATEMENTS OF INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/ConsolidatedStatementsOfIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>030000 - Statement - CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/ConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>040000 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/ConsolidatedStatementsOfChangesInShareholdersEquity</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>040100 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/ConsolidatedStatementsOfChangesInShareholdersEquityParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>050000 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/ConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>050100 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/ConsolidatedStatementsOfCashFlowsParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>060100 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/SummaryOfSignificantAccountingPolicies</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>060200 - Disclosure - ACQUISITIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/Acquisitions</Role>
      <ShortName>ACQUISITIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>060300 - Disclosure - INVESTMENT SECURITIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/InvestmentSecurities</Role>
      <ShortName>INVESTMENT SECURITIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>060400 - Disclosure - LOANS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/Loans</Role>
      <ShortName>LOANS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>060500 - Disclosure - PREMISES AND EQUIPMENT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/PremisesAndEquipment</Role>
      <ShortName>PREMISES AND EQUIPMENT</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>060600 - Disclosure - GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/GoodwillAndIdentifiableIntangibleAssets</Role>
      <ShortName>GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>060700 - Disclosure - DEPOSITS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/Deposits</Role>
      <ShortName>DEPOSITS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>060800 - Disclosure - BORROWINGS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/Borrowings</Role>
      <ShortName>BORROWINGS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>060900 - Disclosure - INCOME TAXES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/IncomeTaxes</Role>
      <ShortName>INCOME TAXES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>061000 - Disclosure - LIMITS ON DIVIDENDS AND OTHER REVENUE SOURCES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/LimitsOnDividendsAndOtherRevenueSources</Role>
      <ShortName>LIMITS ON DIVIDENDS AND OTHER REVENUE SOURCES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>061100 - Disclosure - BENEFIT PLANS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlans</Role>
      <ShortName>BENEFIT PLANS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>061200 - Disclosure - STOCK-BASED COMPENSATION PLANS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/StockbasedCompensationPlans</Role>
      <ShortName>STOCK-BASED COMPENSATION PLANS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>061300 - Disclosure - EARNINGS PER SHARE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/EarningsPerShare</Role>
      <ShortName>EARNINGS PER SHARE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>061400 - Disclosure - COMMITMENTS, CONTINGENT LIABILITIES AND RESTRICTIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/CommitmentsContingentLiabilitiesAndRestrictions</Role>
      <ShortName>COMMITMENTS, CONTINGENT LIABILITIES AND RESTRICTIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>061500 - Disclosure - LEASES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/Leases</Role>
      <ShortName>LEASES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>061600 - Disclosure - REGULATORY MATTERS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/RegulatoryMatters</Role>
      <ShortName>REGULATORY MATTERS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>061700 - Disclosure - PARENT COMPANY STATEMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/ParentCompanyStatements</Role>
      <ShortName>PARENT COMPANY STATEMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>061800 - Disclosure - FAIR VALUE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/FairValue</Role>
      <ShortName>FAIR VALUE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>061900 - Disclosure - DERIVATIVE INSTRUMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/DerivativeInstruments</Role>
      <ShortName>DERIVATIVE INSTRUMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>062000 - Disclosure - VARIABLE INTEREST ENTITIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/VariableInterestEntities</Role>
      <ShortName>VARIABLE INTEREST ENTITIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>062100 - Disclosure - SEGMENT INFORMATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/SegmentInformation</Role>
      <ShortName>SEGMENT INFORMATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>062200 - Disclosure - SUBSEQUENT EVENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/SubsequentEvents</Role>
      <ShortName>SUBSEQUENT EVENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>070100 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/SummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>080200 - Disclosure - ACQUISITIONS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/AcquisitionsTables</Role>
      <ShortName>ACQUISITIONS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/Acquisitions</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>080300 - Disclosure - INVESTMENT SECURITIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/InvestmentSecuritiesTables</Role>
      <ShortName>INVESTMENT SECURITIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/InvestmentSecurities</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>080400 - Disclosure - LOANS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/LoansTables</Role>
      <ShortName>LOANS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/Loans</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>080500 - Disclosure - PREMISES AND EQUIPMENT (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/PremisesAndEquipmentTables</Role>
      <ShortName>PREMISES AND EQUIPMENT (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/PremisesAndEquipment</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>080600 - Disclosure - GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/GoodwillAndIdentifiableIntangibleAssetsTables</Role>
      <ShortName>GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/GoodwillAndIdentifiableIntangibleAssets</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>080700 - Disclosure - DEPOSITS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/DepositsTables</Role>
      <ShortName>DEPOSITS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/Deposits</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>080801 - Disclosure - BORROWINGS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BorrowingsTables</Role>
      <ShortName>BORROWINGS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/Borrowings</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>080900 - Disclosure - INCOME TAXES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/IncomeTaxesTables</Role>
      <ShortName>INCOME TAXES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/IncomeTaxes</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>081100 - Disclosure - BENEFIT PLANS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansTables</Role>
      <ShortName>BENEFIT PLANS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/BenefitPlans</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>081200 - Disclosure - STOCK-BASED COMPENSATION PLANS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/StockbasedCompensationPlansTables</Role>
      <ShortName>STOCK-BASED COMPENSATION PLANS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/StockbasedCompensationPlans</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>081300 - Disclosure - EARNINGS PER SHARE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/EarningsPerShareTables</Role>
      <ShortName>EARNINGS PER SHARE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/EarningsPerShare</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>081400 - Disclosure - COMMITMENTS, CONTINGENT LIABILITIES AND RESTRICTIONS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/CommitmentsContingentLiabilitiesAndRestrictionsTables</Role>
      <ShortName>COMMITMENTS, CONTINGENT LIABILITIES AND RESTRICTIONS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/CommitmentsContingentLiabilitiesAndRestrictions</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>081500 - Disclosure - LEASES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/LeasesTables</Role>
      <ShortName>LEASES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/Leases</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>081600 - Disclosure - REGULATORY MATTERS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/RegulatoryMattersTables</Role>
      <ShortName>REGULATORY MATTERS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/RegulatoryMatters</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>081700 - Disclosure - PARENT COMPANY STATEMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/ParentCompanyStatementsTables</Role>
      <ShortName>PARENT COMPANY STATEMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/ParentCompanyStatements</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>081800 - Disclosure - FAIR VALUE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/FairValueTables</Role>
      <ShortName>FAIR VALUE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/FairValue</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>082000 - Disclosure - VARIABLE INTEREST ENTITIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/VariableInterestEntitiesTables</Role>
      <ShortName>VARIABLE INTEREST ENTITIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/VariableInterestEntities</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>082100 - Disclosure - SEGMENT INFORMATION (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/SegmentInformationTables</Role>
      <ShortName>SEGMENT INFORMATION (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://communitybankna.com/role/SegmentInformation</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>090100 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/SummaryOfSignificantAccountingPoliciesDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/SummaryOfSignificantAccountingPoliciesPolicies</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>090200 - Disclosure - ACQUISITIONS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/AcquisitionsDetails</Role>
      <ShortName>ACQUISITIONS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/AcquisitionsTables</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>090202 - Disclosure - ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/AcquisitionsEstimatedFairValueOfAssetsAcquiredAndLiabilitiesAssumedDetails</Role>
      <ShortName>ACQUISITIONS, Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>090204 - Disclosure - ACQUISITIONS, Summary of Loans Acquired (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/AcquisitionsSummaryOfLoansAcquiredDetails</Role>
      <ShortName>ACQUISITIONS, Summary of Loans Acquired (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>090206 - Disclosure - ACQUISITIONS, Intangible Asset, Goodwill and Acquisition-related Expenses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/AcquisitionsIntangibleAssetGoodwillAndAcquisitionrelatedExpensesDetails</Role>
      <ShortName>ACQUISITIONS, Intangible Asset, Goodwill and Acquisition-related Expenses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>090208 - Disclosure - ACQUISITIONS, Supplemental Pro Forma Financial Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/AcquisitionsSupplementalProFormaFinancialInformationDetails</Role>
      <ShortName>ACQUISITIONS, Supplemental Pro Forma Financial Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>090300 - Disclosure - INVESTMENT SECURITIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/InvestmentSecuritiesDetails</Role>
      <ShortName>INVESTMENT SECURITIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>090302 - Disclosure - INVESTMENT SECURITIES, Investment Securities in a Continuous Unrealized Loss Position (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/InvestmentSecuritiesInvestmentSecuritiesInContinuousUnrealizedLossPositionDetails</Role>
      <ShortName>INVESTMENT SECURITIES, Investment Securities in a Continuous Unrealized Loss Position (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>090304 - Disclosure - INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/InvestmentSecuritiesAmortizedCostAndEstimatedFairValueOfDebtSecuritiesByContractualMaturityDetails</Role>
      <ShortName>INVESTMENT SECURITIES, Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>090306 - Disclosure - INVESTMENT SECURITIES, Cash Flow Information on Investment Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/InvestmentSecuritiesCashFlowInformationOnInvestmentSecuritiesDetails</Role>
      <ShortName>INVESTMENT SECURITIES, Cash Flow Information on Investment Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>090400 - Disclosure - LOANS, Loan Summary (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/LoansLoanSummaryDetails</Role>
      <ShortName>LOANS, Loan Summary (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>090402 - Disclosure - LOANS, Acquired Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/LoansAcquiredLoansDetails</Role>
      <ShortName>LOANS, Acquired Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>090404 - Disclosure - LOANS, Credit Quality By Past Due Status (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/LoansCreditQualityByPastDueStatusDetails</Role>
      <ShortName>LOANS, Credit Quality By Past Due Status (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>090406 - Disclosure - LOANS, Amount of Business Lending Loans by Credit Quality Category (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/LoansAmountOfBusinessLendingLoansByCreditQualityCategoryDetails</Role>
      <ShortName>LOANS, Amount of Business Lending Loans by Credit Quality Category (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>090408 - Disclosure - LOANS, Summary of Impaired Loans, Excluding Purchased Impaired (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/LoansSummaryOfImpairedLoansExcludingPurchasedImpairedDetails</Role>
      <ShortName>LOANS, Summary of Impaired Loans, Excluding Purchased Impaired (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>090410 - Disclosure - LOANS, Troubled Debt Restructuring (TDR) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/LoansTroubledDebtRestructuringTdrDetails</Role>
      <ShortName>LOANS, Troubled Debt Restructuring (TDR) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>090412 - Disclosure - LOANS, Allowance for Loan Losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/LoansAllowanceForLoanLossesDetails</Role>
      <ShortName>LOANS, Allowance for Loan Losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>090500 - Disclosure - PREMISES AND EQUIPMENT (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/PremisesAndEquipmentDetails</Role>
      <ShortName>PREMISES AND EQUIPMENT (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/PremisesAndEquipmentTables</ParentRole>
      <Position>68</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>090600 - Disclosure - GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/GoodwillAndIdentifiableIntangibleAssetsDetails</Role>
      <ShortName>GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/GoodwillAndIdentifiableIntangibleAssetsTables</ParentRole>
      <Position>69</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>090700 - Disclosure - DEPOSITS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/DepositsDetails</Role>
      <ShortName>DEPOSITS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/DepositsTables</ParentRole>
      <Position>70</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>090800 - Disclosure - BORROWINGS, Outstanding Borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BorrowingsOutstandingBorrowingsDetails</Role>
      <ShortName>BORROWINGS, Outstanding Borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>090804 - Disclosure - BORROWINGS, Contractual Maturity on Borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BorrowingsContractualMaturityOnBorrowingsDetails</Role>
      <ShortName>BORROWINGS, Contractual Maturity on Borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>090806 - Disclosure - BORROWINGS, Preferred Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BorrowingsPreferredSecuritiesDetails</Role>
      <ShortName>BORROWINGS, Preferred Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>090900 - Disclosure - INCOME TAXES, Provision for Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/IncomeTaxesProvisionForIncomeTaxesDetails</Role>
      <ShortName>INCOME TAXES, Provision for Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>090902 - Disclosure - INCOME TAXES, Components of Net Deferred Tax Liability (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/IncomeTaxesComponentsOfNetDeferredTaxLiabilityDetails</Role>
      <ShortName>INCOME TAXES, Components of Net Deferred Tax Liability (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>090904 - Disclosure - INCOME TAXES, Effective Tax Rate Reconciliation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/IncomeTaxesEffectiveTaxRateReconciliationDetails</Role>
      <ShortName>INCOME TAXES, Effective Tax Rate Reconciliation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>090906 - Disclosure - INCOME TAXES, Unrecognized Tax Benefits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/IncomeTaxesUnrecognizedTaxBenefitsDetails</Role>
      <ShortName>INCOME TAXES, Unrecognized Tax Benefits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>091000 - Disclosure - LIMITS ON DIVIDENDS AND OTHER REVENUE SOURCES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/LimitsOnDividendsAndOtherRevenueSourcesDetails</Role>
      <ShortName>LIMITS ON DIVIDENDS AND OTHER REVENUE SOURCES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/LimitsOnDividendsAndOtherRevenueSources</ParentRole>
      <Position>78</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>091100 - Disclosure - BENEFIT PLANS, Funded Status and Amounts Recognized in Balance Sheets and AOCI (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansFundedStatusAndAmountsRecognizedInBalanceSheetsAndAociDetails</Role>
      <ShortName>BENEFIT PLANS, Funded Status and Amounts Recognized in Balance Sheets and AOCI (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>091102 - Disclosure - BENEFIT PLANS, Amounts Recognized in AOCI, Net of Tax (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansAmountsRecognizedInAociNetOfTaxDetails</Role>
      <ShortName>BENEFIT PLANS, Amounts Recognized in AOCI, Net of Tax (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>091104 - Disclosure - BENEFIT PLANS, Costs That Will Be Amortized from AOCI in Next Fiscal Year (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansCostsThatWillBeAmortizedFromAociInNextFiscalYearDetails</Role>
      <ShortName>BENEFIT PLANS, Costs That Will Be Amortized from AOCI in Next Fiscal Year (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>091106 - Disclosure - BENEFIT PLANS, Weighted-average Assumptions Used to Determine Benefit Obligations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansWeightedaverageAssumptionsUsedToDetermineBenefitObligationsDetails</Role>
      <ShortName>BENEFIT PLANS, Weighted-average Assumptions Used to Determine Benefit Obligations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>091108 - Disclosure - BENEFIT PLANS, Net Periodic Benefit Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansNetPeriodicBenefitCostDetails</Role>
      <ShortName>BENEFIT PLANS, Net Periodic Benefit Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>091110 - Disclosure - BENEFIT PLANS, Weighted-average Assumptions Used to Determine Net Periodic Benefit Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansWeightedaverageAssumptionsUsedToDetermineNetPeriodicBenefitCostDetails</Role>
      <ShortName>BENEFIT PLANS, Weighted-average Assumptions Used to Determine Net Periodic Benefit Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>091112 - Disclosure - BENEFIT PLANS, Estimated Future Benefit Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansEstimatedFutureBenefitPaymentsDetails</Role>
      <ShortName>BENEFIT PLANS, Estimated Future Benefit Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>091114 - Disclosure - BENEFIT PLANS, Target Plan Assets Allocation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansTargetPlanAssetsAllocationDetails</Role>
      <ShortName>BENEFIT PLANS, Target Plan Assets Allocation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>091116 - Disclosure - BENEFIT PLANS, Fair Value of Plan Assets by Assets Category (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansFairValueOfPlanAssetsByAssetsCategoryDetails</Role>
      <ShortName>BENEFIT PLANS, Fair Value of Plan Assets by Assets Category (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>091118 - Disclosure - BENEFIT PLANS, Multi-employer Defined Benefit Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansMultiemployerDefinedBenefitPlanDetails</Role>
      <ShortName>BENEFIT PLANS, Multi-employer Defined Benefit Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>091120 - Disclosure - BENEFIT PLANS, Assumed Health Care Cost Trend Rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansAssumedHealthCareCostTrendRateDetails</Role>
      <ShortName>BENEFIT PLANS, Assumed Health Care Cost Trend Rate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>091122 - Disclosure - BENEFIT PLANS, 401(K) Employee Stock Ownership Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlans401KEmployeeStockOwnershipPlanDetails</Role>
      <ShortName>BENEFIT PLANS, 401(K) Employee Stock Ownership Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>091124 - Disclosure - BENEFIT PLANS, Other Deferred Compensation Arrangements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansOtherDeferredCompensationArrangementsDetails</Role>
      <ShortName>BENEFIT PLANS, Other Deferred Compensation Arrangements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>091126 - Disclosure - BENEFIT PLANS, Deferred Compensation Plan for Directors (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/BenefitPlansDeferredCompensationPlanForDirectorsDetails</Role>
      <ShortName>BENEFIT PLANS, Deferred Compensation Plan for Directors (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>091200 - Disclosure - STOCK-BASED COMPENSATION PLANS, Long-Term Incentive Program (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/StockbasedCompensationPlansLongtermIncentiveProgramDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION PLANS, Long-Term Incentive Program (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>091202 - Disclosure - STOCK-BASED COMPENSATION PLANS, Activity in Long-Term Incentive Program (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/StockbasedCompensationPlansActivityInLongtermIncentiveProgramDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION PLANS, Activity in Long-Term Incentive Program (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>091204 - Disclosure - STOCK-BASED COMPENSATION PLANS, Summary of Stock Options Outstanding (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/StockbasedCompensationPlansSummaryOfStockOptionsOutstandingDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION PLANS, Summary of Stock Options Outstanding (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>091206 - Disclosure - STOCK-BASED COMPENSATION PLANS, Stock-Based Compensation Expense Recognized (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/StockbasedCompensationPlansStockbasedCompensationExpenseRecognizedDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION PLANS, Stock-Based Compensation Expense Recognized (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>091208 - Disclosure - STOCK-BASED COMPENSATION PLANS, Assumptions Used to Estimate Value of Stock Options (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/StockbasedCompensationPlansAssumptionsUsedToEstimateValueOfStockOptionsDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION PLANS, Assumptions Used to Estimate Value of Stock Options (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>091210 - Disclosure - STOCK-BASED COMPENSATION PLANS, Unrecognized Stock-Based Compensation Expense and Stock Option Exercises (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/StockbasedCompensationPlansUnrecognizedStockbasedCompensationExpenseAndStockOptionExercisesDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION PLANS, Unrecognized Stock-Based Compensation Expense and Stock Option Exercises (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>091212 - Disclosure - STOCK-BASED COMPENSATION PLANS, Activity of Unvested Restricted Stock Awards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/StockbasedCompensationPlansActivityOfUnvestedRestrictedStockAwardsDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION PLANS, Activity of Unvested Restricted Stock Awards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>091300 - Disclosure - EARNINGS PER SHARE (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/EarningsPerShareDetails</Role>
      <ShortName>EARNINGS PER SHARE (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/EarningsPerShareTables</ParentRole>
      <Position>100</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>091400 - Disclosure - COMMITMENTS, CONTINGENT LIABILITIES AND RESTRICTIONS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/CommitmentsContingentLiabilitiesAndRestrictionsDetails</Role>
      <ShortName>COMMITMENTS, CONTINGENT LIABILITIES AND RESTRICTIONS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/CommitmentsContingentLiabilitiesAndRestrictionsTables</ParentRole>
      <Position>101</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>091500 - Disclosure - LEASES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/LeasesDetails</Role>
      <ShortName>LEASES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/LeasesTables</ParentRole>
      <Position>102</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>091600 - Disclosure - REGULATORY MATTERS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/RegulatoryMattersDetails</Role>
      <ShortName>REGULATORY MATTERS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/RegulatoryMattersTables</ParentRole>
      <Position>103</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>091700 - Disclosure - PARENT COMPANY STATEMENTS, Condensed Balance Sheets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/ParentCompanyStatementsCondensedBalanceSheetsDetails</Role>
      <ShortName>PARENT COMPANY STATEMENTS, Condensed Balance Sheets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>104</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>091702 - Disclosure - PARENT COMPANY STATEMENTS, Condensed Statements of Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/ParentCompanyStatementsCondensedStatementsOfIncomeDetails</Role>
      <ShortName>PARENT COMPANY STATEMENTS, Condensed Statements of Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>105</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>091704 - Disclosure - PARENT COMPANY STATEMENTS, Condensed Statements of Cash Flows (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/ParentCompanyStatementsCondensedStatementsOfCashFlowsDetails</Role>
      <ShortName>PARENT COMPANY STATEMENTS, Condensed Statements of Cash Flows (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>106</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>091800 - Disclosure - FAIR VALUE, Financial Assets and Liabilities Accounted for at Fair Value on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/FairValueFinancialAssetsAndLiabilitiesAccountedForAtFairValueOnRecurringBasisDetails</Role>
      <ShortName>FAIR VALUE, Financial Assets and Liabilities Accounted for at Fair Value on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>107</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R108.htm</HtmlFileName>
      <LongName>091802 - Disclosure - FAIR VALUE, Unobservable Input Reconciliation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/FairValueUnobservableInputReconciliationDetails</Role>
      <ShortName>FAIR VALUE, Unobservable Input Reconciliation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>108</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R109.htm</HtmlFileName>
      <LongName>091804 - Disclosure - FAIR VALUE, Assets and Liabilities Measured on Nonrecurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/FairValueAssetsAndLiabilitiesMeasuredOnNonrecurringBasisDetails</Role>
      <ShortName>FAIR VALUE, Assets and Liabilities Measured on Nonrecurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>109</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R110.htm</HtmlFileName>
      <LongName>091806 - Disclosure - FAIR VALUE, Significant Unobservable Inputs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/FairValueSignificantUnobservableInputsDetails</Role>
      <ShortName>FAIR VALUE, Significant Unobservable Inputs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>110</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R111.htm</HtmlFileName>
      <LongName>091808 - Disclosure - FAIR VALUE, Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/FairValueCarryingAmountsAndEstimatedFairValuesOfOtherFinancialInstrumentsDetails</Role>
      <ShortName>FAIR VALUE, Carrying Amounts and Estimated Fair Values of Other Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>111</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R112.htm</HtmlFileName>
      <LongName>092000 - Disclosure - VARIABLE INTEREST ENTITIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/VariableInterestEntitiesDetails</Role>
      <ShortName>VARIABLE INTEREST ENTITIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/VariableInterestEntitiesTables</ParentRole>
      <Position>112</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R113.htm</HtmlFileName>
      <LongName>092100 - Disclosure - SEGMENT INFORMATION (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/SegmentInformationDetails</Role>
      <ShortName>SEGMENT INFORMATION (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/SegmentInformationTables</ParentRole>
      <Position>113</Position>
    </Report>
    <Report instance="cbu-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R114.htm</HtmlFileName>
      <LongName>092200 - Disclosure - SUBSEQUENT EVENTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://communitybankna.com/role/SubsequentEventsDetails</Role>
      <ShortName>SUBSEQUENT EVENTS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://communitybankna.com/role/SubsequentEvents</ParentRole>
      <Position>114</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>cbu-20161231.xml</File>
    <File>cbu-20161231.xsd</File>
    <File>cbu-20161231_cal.xml</File>
    <File>cbu-20161231_def.xml</File>
    <File>cbu-20161231_lab.xml</File>
    <File>cbu-20161231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies/>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
