<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.6.0.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>447</ContextCount>
  <ElementCount>570</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>104</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>5</UnitCount>
  <MyReports>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>101 - Document - Document And Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/DocumentAndEntityInformation</Role>
      <ShortName>Document And Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>102 - Statement - CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>103 - Statement - CONSOLIDATED BALANCE SHEETS [Parenthetical]</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS [Parenthetical]</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>104 - Statement - CONSOLIDATED STATEMENTS OF INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ConsolidatedStatementsOfIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>105 - Statement - CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>106 - Statement - CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME [Parenthetical]</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ConsolidatedStatementsOfComprehensiveIncomeParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME [Parenthetical]</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>107 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ConsolidatedStatementsOfChangesInStockholdersEquity</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>108 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY [Parenthetical]</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ConsolidatedStatementsOfChangesInStockholdersEquityParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY [Parenthetical]</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>109 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>110 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/SummaryOfSignificantAccountingPolicies</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>111 - Disclosure - RESTRICTED CASH BALANCES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/RestrictedCashBalances</Role>
      <ShortName>RESTRICTED CASH BALANCES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>112 - Disclosure - INVESTMENT SECURITIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/InvestmentSecurities</Role>
      <ShortName>INVESTMENT SECURITIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>113 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLosses</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>114 - Disclosure - MORTGAGE SERVICING RIGHTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/MortgageServicingRights</Role>
      <ShortName>MORTGAGE SERVICING RIGHTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>115 - Disclosure - PREMISES AND EQUIPMENT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/PremisesAndEquipment</Role>
      <ShortName>PREMISES AND EQUIPMENT</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>116 - Disclosure - CORE DEPOSIT INTANGIBLES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/CoreDepositIntangibles</Role>
      <ShortName>CORE DEPOSIT INTANGIBLES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>117 - Disclosure - DEPOSITS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/Deposits</Role>
      <ShortName>DEPOSITS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>118 - Disclosure - SHORT-TERM BORROWINGS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ShorttermBorrowings</Role>
      <ShortName>SHORT-TERM BORROWINGS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>119 - Disclosure - LONG-TERM BORROWING</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LongtermBorrowing</Role>
      <ShortName>LONG-TERM BORROWING</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>120 - Disclosure - INCOME TAXES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/IncomeTaxes</Role>
      <ShortName>INCOME TAXES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>121 - Disclosure - EMPLOYEE BENEFIT PLANS AND DEFERRED COMPENSATION AGREEMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/EmployeeBenefitPlansAndDeferredCompensationAgreements</Role>
      <ShortName>EMPLOYEE BENEFIT PLANS AND DEFERRED COMPENSATION AGREEMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>122 - Disclosure - COMMITMENTS AND CONTINGENCIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/CommitmentsAndContingencies</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>123 - Disclosure - RELATED PARTY TRANSACTIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/RelatedPartyTransactions</Role>
      <ShortName>RELATED PARTY TRANSACTIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>124 - Disclosure - REGULATORY MATTERS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/RegulatoryMatters</Role>
      <ShortName>REGULATORY MATTERS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>125 - Disclosure - FINANCIAL INSTRUMENTS WITH OFF-BALANCE SHEET RISK AND CONCENTRATIONS OF CREDIT RISK</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/FinancialInstrumentsWithOffbalanceSheetRiskAndConcentrationsOfCreditRisk</Role>
      <ShortName>FINANCIAL INSTRUMENTS WITH OFF-BALANCE SHEET RISK AND CONCENTRATIONS OF CREDIT RISK</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>126 - Disclosure - STOCKHOLDERS' EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/StockholdersEquity</Role>
      <ShortName>STOCKHOLDERS' EQUITY</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>127 - Disclosure - STOCK COMPENSATION PLAN</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/StockCompensationPlan</Role>
      <ShortName>STOCK COMPENSATION PLAN</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>128 - Disclosure - FAIR VALUE MEASUREMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/FairValueMeasurements</Role>
      <ShortName>FAIR VALUE MEASUREMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>129 - Disclosure - PARENT COMPANY FINANCIAL INFORMATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ParentCompanyFinancialInformation</Role>
      <ShortName>PARENT COMPANY FINANCIAL INFORMATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>130 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/SummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>131 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/SummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/SummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>132 - Disclosure - INVESTMENT SECURITIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/InvestmentSecuritiesTables</Role>
      <ShortName>INVESTMENT SECURITIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/InvestmentSecurities</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>133 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesTables</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLosses</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>134 - Disclosure - MORTGAGE SERVICING RIGHTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/MortgageServicingRightsTables</Role>
      <ShortName>MORTGAGE SERVICING RIGHTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/MortgageServicingRights</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>135 - Disclosure - PREMISES AND EQUIPMENT (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/PremisesAndEquipmentTables</Role>
      <ShortName>PREMISES AND EQUIPMENT (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/PremisesAndEquipment</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>136 - Disclosure - CORE DEPOSIT INTANGIBLES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/CoreDepositIntangiblesTables</Role>
      <ShortName>CORE DEPOSIT INTANGIBLES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/CoreDepositIntangibles</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>137 - Disclosure - DEPOSITS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/DepositsTables</Role>
      <ShortName>DEPOSITS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/Deposits</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>138 - Disclosure - SHORT-TERM BORROWINGS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ShorttermBorrowingsTables</Role>
      <ShortName>SHORT-TERM BORROWINGS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/ShorttermBorrowings</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>139 - Disclosure - LONG-TERM BORROWINGS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LongtermBorrowingsTables</Role>
      <ShortName>LONG-TERM BORROWINGS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LongtermBorrowing</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>140 - Disclosure - INCOME TAXES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/IncomeTaxesTables</Role>
      <ShortName>INCOME TAXES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/IncomeTaxes</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>141 - Disclosure - COMMITMENTS AND CONTINGENCIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/CommitmentsAndContingenciesTables</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/CommitmentsAndContingencies</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>142 - Disclosure - RELATED PARTY TRANSACTIONS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/RelatedPartyTransactionsTables</Role>
      <ShortName>RELATED PARTY TRANSACTIONS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/RelatedPartyTransactions</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>143 - Disclosure - REGULATORY MATTERS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/RegulatoryMattersTables</Role>
      <ShortName>REGULATORY MATTERS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/RegulatoryMatters</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>144 - Disclosure - FINANCIAL INSTRUMENTS WITH OFF-BALANCE SHEET RISK AND CONCENTRATIONS OF CREDIT RISK (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/FinancialInstrumentsWithOffbalanceSheetRiskAndConcentrationsOfCreditRiskTables</Role>
      <ShortName>FINANCIAL INSTRUMENTS WITH OFF-BALANCE SHEET RISK AND CONCENTRATIONS OF CREDIT RISK (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/FinancialInstrumentsWithOffbalanceSheetRiskAndConcentrationsOfCreditRisk</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>145 - Disclosure - STOCK COMPENSATION PLAN (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/StockCompensationPlanTables</Role>
      <ShortName>STOCK COMPENSATION PLAN (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/StockCompensationPlan</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>146 - Disclosure - FAIR VALUE MEASUREMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/FairValueMeasurementsTables</Role>
      <ShortName>FAIR VALUE MEASUREMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/FairValueMeasurements</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>147 - Disclosure - PARENT COMPANY FINANCIAL INFORMATION (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ParentCompanyFinancialInformationTables</Role>
      <ShortName>PARENT COMPANY FINANCIAL INFORMATION (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/ParentCompanyFinancialInformation</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>148 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/SummaryOfSignificantAccountingPoliciesDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/SummaryOfSignificantAccountingPoliciesTables</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>149 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/SummaryOfSignificantAccountingPoliciesDetailsTextual</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/SummaryOfSignificantAccountingPoliciesTables</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>150 - Disclosure - RESTRICTED CASH BALANCES (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/RestrictedCashBalancesDetailsTextual</Role>
      <ShortName>RESTRICTED CASH BALANCES (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/RestrictedCashBalances</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>151 - Disclosure - INVESTMENT SECURITIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/InvestmentSecuritiesDetails</Role>
      <ShortName>INVESTMENT SECURITIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>152 - Disclosure - INVESTMENT SECURITIES (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/InvestmentSecuritiesDetails1</Role>
      <ShortName>INVESTMENT SECURITIES (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>153 - Disclosure - INVESTMENT SECURITIES (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/InvestmentSecuritiesDetails2</Role>
      <ShortName>INVESTMENT SECURITIES (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>154 - Disclosure - INVESTMENT SECURITIES (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/InvestmentSecuritiesDetailsTextual</Role>
      <ShortName>INVESTMENT SECURITIES (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>54</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>155 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesDetails</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesTables</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>156 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesDetails1</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesTables</ParentRole>
      <Position>56</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>157 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesDetails2</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesTables</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>158 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 3)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesDetails3</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 3)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesTables</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>159 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 4)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesDetails4</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 4)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesTables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>160 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 5)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesDetails5</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 5)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesTables</ParentRole>
      <Position>60</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>161 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 6)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesDetails6</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 6)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesTables</ParentRole>
      <Position>61</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>162 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 7)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesDetails7</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES (Details 7)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesTables</ParentRole>
      <Position>62</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>163 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesDetailsTextual</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LoansAndAllowanceForLoanLossesTables</ParentRole>
      <Position>63</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>164 - Disclosure - MORTGAGE SERVICING RIGHTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/MortgageServicingRightsDetails</Role>
      <ShortName>MORTGAGE SERVICING RIGHTS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/MortgageServicingRightsTables</ParentRole>
      <Position>64</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>165 - Disclosure - MORTGAGE SERVICING RIGHTS (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/MortgageServicingRightsDetailsTextual</Role>
      <ShortName>MORTGAGE SERVICING RIGHTS (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/MortgageServicingRightsTables</ParentRole>
      <Position>65</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>166 - Disclosure - PREMISES AND EQUIPMENT (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/PremisesAndEquipmentDetails</Role>
      <ShortName>PREMISES AND EQUIPMENT (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/PremisesAndEquipmentTables</ParentRole>
      <Position>66</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>167 - Disclosure - PREMISES AND EQUIPMENT (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/PremisesAndEquipmentDetailsTextual</Role>
      <ShortName>PREMISES AND EQUIPMENT (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/PremisesAndEquipmentTables</ParentRole>
      <Position>67</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>168 - Disclosure - CORE DEPOSIT INTANGIBLES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/CoreDepositIntangiblesDetails</Role>
      <ShortName>CORE DEPOSIT INTANGIBLES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/CoreDepositIntangiblesTables</ParentRole>
      <Position>68</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>169 - Disclosure - CORE DEPOSIT INTANGIBLES (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/CoreDepositIntangiblesDetailsTextual</Role>
      <ShortName>CORE DEPOSIT INTANGIBLES (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/CoreDepositIntangiblesTables</ParentRole>
      <Position>69</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>170 - Disclosure - DEPOSITS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/DepositsDetails</Role>
      <ShortName>DEPOSITS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/DepositsTables</ParentRole>
      <Position>70</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>171 - Disclosure - DEPOSITS (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/DepositsDetails1</Role>
      <ShortName>DEPOSITS (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/DepositsTables</ParentRole>
      <Position>71</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>172 - Disclosure - DEPOSITS (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/DepositsDetails2</Role>
      <ShortName>DEPOSITS (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/DepositsTables</ParentRole>
      <Position>72</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>173 - Disclosure - SHORT-TERM BORROWINGS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ShorttermBorrowingsDetails</Role>
      <ShortName>SHORT-TERM BORROWINGS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/ShorttermBorrowingsTables</ParentRole>
      <Position>73</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>174 - Disclosure - SHORT-TERM BORROWINGS (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ShorttermBorrowingsDetails1</Role>
      <ShortName>SHORT-TERM BORROWINGS (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/ShorttermBorrowingsTables</ParentRole>
      <Position>74</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>175 - Disclosure - SHORT-TERM BORROWINGS (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ShorttermBorrowingsDetails2</Role>
      <ShortName>SHORT-TERM BORROWINGS (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/ShorttermBorrowingsTables</ParentRole>
      <Position>75</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>176 - Disclosure - SHORT-TERM BORROWINGS (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ShorttermBorrowingsDetailsTextual</Role>
      <ShortName>SHORT-TERM BORROWINGS (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/ShorttermBorrowingsTables</ParentRole>
      <Position>76</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>177 - Disclosure - LONG-TERM BORROWINGS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LongtermBorrowingsDetails</Role>
      <ShortName>LONG-TERM BORROWINGS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LongtermBorrowingsTables</ParentRole>
      <Position>77</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>178 - Disclosure - LONG-TERM BORROWINGS (Details) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LongtermBorrowingsDetailsParenthetical</Role>
      <ShortName>LONG-TERM BORROWINGS (Details) (Parenthetical)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LongtermBorrowingsTables</ParentRole>
      <Position>78</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>179 - Disclosure - LONG-TERM BORROWINGS (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/LongtermBorrowingsDetailsTextual</Role>
      <ShortName>LONG-TERM BORROWINGS (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/LongtermBorrowingsTables</ParentRole>
      <Position>79</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>180 - Disclosure - INCOME TAXES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/IncomeTaxesDetails</Role>
      <ShortName>INCOME TAXES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/IncomeTaxesTables</ParentRole>
      <Position>80</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>181 - Disclosure - INCOME TAXES (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/IncomeTaxesDetails1</Role>
      <ShortName>INCOME TAXES (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/IncomeTaxesTables</ParentRole>
      <Position>81</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>182 - Disclosure - INCOME TAXES (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/IncomeTaxesDetails2</Role>
      <ShortName>INCOME TAXES (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/IncomeTaxesTables</ParentRole>
      <Position>82</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>183 - Disclosure - INCOME TAXES (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/IncomeTaxesDetailsTextual</Role>
      <ShortName>INCOME TAXES (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/IncomeTaxesTables</ParentRole>
      <Position>83</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>184 - Disclosure - EMPLOYEE BENEFIT PLANS AND DEFERRED COMPENSATION AGREEMENTS (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/EmployeeBenefitPlansAndDeferredCompensationAgreementsDetailsTextual</Role>
      <ShortName>EMPLOYEE BENEFIT PLANS AND DEFERRED COMPENSATION AGREEMENTS (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/EmployeeBenefitPlansAndDeferredCompensationAgreements</ParentRole>
      <Position>84</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>185 - Disclosure - COMMITMENTS AND CONTINGENCIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/CommitmentsAndContingenciesDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/CommitmentsAndContingenciesTables</ParentRole>
      <Position>85</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>186 - Disclosure - COMMITMENTS AND CONTINGENCIES (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/CommitmentsAndContingenciesDetailsTextual</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/CommitmentsAndContingenciesTables</ParentRole>
      <Position>86</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>187 - Disclosure - RELATED PARTY TRANSACTIONS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/RelatedPartyTransactionsDetails</Role>
      <ShortName>RELATED PARTY TRANSACTIONS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/RelatedPartyTransactionsTables</ParentRole>
      <Position>87</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>188 - Disclosure - RELATED PARTY TRANSACTIONS (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/RelatedPartyTransactionsDetailsTextual</Role>
      <ShortName>RELATED PARTY TRANSACTIONS (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/RelatedPartyTransactionsTables</ParentRole>
      <Position>88</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>189 - Disclosure - REGULATORY MATTERS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/RegulatoryMattersDetails</Role>
      <ShortName>REGULATORY MATTERS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/RegulatoryMattersTables</ParentRole>
      <Position>89</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>190 - Disclosure - REGULATORY MATTERS (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/RegulatoryMattersDetailsTextual</Role>
      <ShortName>REGULATORY MATTERS (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/RegulatoryMattersTables</ParentRole>
      <Position>90</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>191 - Disclosure - FINANCIAL INSTRUMENTS WITH OFF-BALANCE SHEET RISK AND CONCENTRATIONS OF CREDIT RISK (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/FinancialInstrumentsWithOffbalanceSheetRiskAndConcentrationsOfCreditRiskDetails</Role>
      <ShortName>FINANCIAL INSTRUMENTS WITH OFF-BALANCE SHEET RISK AND CONCENTRATIONS OF CREDIT RISK (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/FinancialInstrumentsWithOffbalanceSheetRiskAndConcentrationsOfCreditRiskTables</ParentRole>
      <Position>91</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>192 - Disclosure - FINANCIAL INSTRUMENTS WITH OFF-BALANCE SHEET RISK AND CONCENTRATIONS OF CREDIT RISK (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/FinancialInstrumentsWithOffbalanceSheetRiskAndConcentrationsOfCreditRiskDetailsTextual</Role>
      <ShortName>FINANCIAL INSTRUMENTS WITH OFF-BALANCE SHEET RISK AND CONCENTRATIONS OF CREDIT RISK (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/FinancialInstrumentsWithOffbalanceSheetRiskAndConcentrationsOfCreditRiskTables</ParentRole>
      <Position>92</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>193 - Disclosure - STOCKHOLDERS' EQUITY (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/StockholdersEquityDetailsTextual</Role>
      <ShortName>STOCKHOLDERS' EQUITY (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/StockholdersEquity</ParentRole>
      <Position>93</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>194 - Disclosure - STOCK COMPENSATION PLAN (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/StockCompensationPlanDetails</Role>
      <ShortName>STOCK COMPENSATION PLAN (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/StockCompensationPlanTables</ParentRole>
      <Position>94</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>195 - Disclosure - STOCK COMPENSATION PLAN (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/StockCompensationPlanDetails1</Role>
      <ShortName>STOCK COMPENSATION PLAN (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/StockCompensationPlanTables</ParentRole>
      <Position>95</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>196 - Disclosure - STOCK COMPENSATION PLAN (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/StockCompensationPlanDetailsTextual</Role>
      <ShortName>STOCK COMPENSATION PLAN (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/StockCompensationPlanTables</ParentRole>
      <Position>96</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>197 - Disclosure - FAIR VALUE MEASUREMENTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/FairValueMeasurementsDetails</Role>
      <ShortName>FAIR VALUE MEASUREMENTS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>97</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>198 - Disclosure - FAIR VALUE MEASUREMENTS (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/FairValueMeasurementsDetails1</Role>
      <ShortName>FAIR VALUE MEASUREMENTS (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>98</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>199 - Disclosure - FAIR VALUE MEASUREMENTS (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/FairValueMeasurementsDetails2</Role>
      <ShortName>FAIR VALUE MEASUREMENTS (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>99</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>200 - Disclosure - FAIR VALUE MEASUREMENTS (Details 3)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/FairValueMeasurementsDetails3</Role>
      <ShortName>FAIR VALUE MEASUREMENTS (Details 3)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>100</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>201 - Disclosure - FAIR VALUE MEASUREMENTS (Details 4)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/FairValueMeasurementsDetails4</Role>
      <ShortName>FAIR VALUE MEASUREMENTS (Details 4)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>101</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>202 - Disclosure - FAIR VALUE MEASUREMENTS (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/FairValueMeasurementsDetailsTextual</Role>
      <ShortName>FAIR VALUE MEASUREMENTS (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>102</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>203 - Disclosure - PARENT COMPANY FINANCIAL INFORMATION (BALANCE SHEETS) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ParentCompanyFinancialInformationBalanceSheetsDetails</Role>
      <ShortName>PARENT COMPANY FINANCIAL INFORMATION (BALANCE SHEETS) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/ParentCompanyFinancialInformationTables</ParentRole>
      <Position>103</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>204 - Disclosure - PARENT COMPANY FINANCIAL INFORMATION (STATEMENTS OF INCOME) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ParentCompanyFinancialInformationStatementsOfIncomeDetails</Role>
      <ShortName>PARENT COMPANY FINANCIAL INFORMATION (STATEMENTS OF INCOME) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/ParentCompanyFinancialInformationTables</ParentRole>
      <Position>104</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>205 - Disclosure - PARENT COMPANY FINANCIAL INFORMATION (STATEMENTS OF COMPREHENSIVE INCOME (LOSS)) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ParentCompanyFinancialInformationStatementsOfComprehensiveIncomeLossDetails</Role>
      <ShortName>PARENT COMPANY FINANCIAL INFORMATION (STATEMENTS OF COMPREHENSIVE INCOME (LOSS)) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/ParentCompanyFinancialInformationTables</ParentRole>
      <Position>105</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>206 - Disclosure - PARENT COMPANY FINANCIAL INFORMATION (STATEMENTS OF CASH FLOWS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ParentCompanyFinancialInformationStatementsOfCashFlowsDetails</Role>
      <ShortName>PARENT COMPANY FINANCIAL INFORMATION (STATEMENTS OF CASH FLOWS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/ParentCompanyFinancialInformationTables</ParentRole>
      <Position>106</Position>
    </Report>
    <Report instance="fkys-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>207 - Disclosure - PARENT COMPANY FINANCIAL INFORMATION (STATEMENTS OF COMPREHENSIVE INCOME (LOSS)) (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.firstkeystonecorporation.com/role/ParentCompanyFinancialInformationStatementsOfComprehensiveIncomeLossDetailsTextual</Role>
      <ShortName>PARENT COMPANY FINANCIAL INFORMATION (STATEMENTS OF COMPREHENSIVE INCOME (LOSS)) (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.firstkeystonecorporation.com/role/ParentCompanyFinancialInformationTables</ParentRole>
      <Position>107</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>fkys-20161231.xml</File>
    <File>fkys-20161231.xsd</File>
    <File>fkys-20161231_cal.xml</File>
    <File>fkys-20161231_def.xml</File>
    <File>fkys-20161231_lab.xml</File>
    <File>fkys-20161231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies/>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
