<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.7.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>282</ContextCount>
  <ElementCount>577</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>85</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>10</UnitCount>
  <MyReports>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>100000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>100010 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/StatementConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>100020 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/StatementConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>100030 - Statement - Consolidated Statements of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/StatementConsolidatedStatementsOfOperations</Role>
      <ShortName>Consolidated Statements of Operations</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>100040 - Statement - Consolidated Statements of Comprehensive Loss</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/StatementConsolidatedStatementsOfComprehensiveLoss</Role>
      <ShortName>Consolidated Statements of Comprehensive Loss</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>100060 - Statement - Consolidated Statements Changes in Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/StatementConsolidatedStatementsChangesInStockholdersEquity</Role>
      <ShortName>Consolidated Statements Changes in Stockholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>100070 - Statement - Consolidated Statements Changes in Stockholders' Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/StatementConsolidatedStatementsChangesInStockholdersEquityParenthetical</Role>
      <ShortName>Consolidated Statements Changes in Stockholders' Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>100080 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/StatementConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>100090 - Disclosure - Conversion</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureConversion</Role>
      <ShortName>Conversion</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>100100 - Disclosure - Nature of Operations and Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureNatureOfOperationsAndBasisOfPresentation</Role>
      <ShortName>Nature of Operations and Basis of Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>100110 - Disclosure - Acquisition</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureAcquisition</Role>
      <ShortName>Acquisition</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>100120 - Disclosure - Securities Available for Sale</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSecuritiesAvailableForSale</Role>
      <ShortName>Securities Available for Sale</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>100130 - Disclosure - Loans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoans</Role>
      <ShortName>Loans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>100140 - Disclosure - Loan Servicing</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoanServicing</Role>
      <ShortName>Loan Servicing</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>100150 - Disclosure - Premises and Equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePremisesAndEquipment</Role>
      <ShortName>Premises and Equipment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>100160 - Disclosure - Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureDeposits</Role>
      <ShortName>Deposits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>100170 - Disclosure - Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureBorrowings</Role>
      <ShortName>Borrowings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>100180 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureIncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>100190 - Disclosure - On-Balance Sheet Derivative Instruments and Hedging Activities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureOnBalanceSheetDerivativeInstrumentsAndHedgingActivities</Role>
      <ShortName>On-Balance Sheet Derivative Instruments and Hedging Activities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>100200 - Disclosure - Minimum Regulatory Capital Requirements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureMinimumRegulatoryCapitalRequirements</Role>
      <ShortName>Minimum Regulatory Capital Requirements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>100210 - Disclosure - Pension and Other Post-retirement Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePensionAndOtherPostRetirementPlans</Role>
      <ShortName>Pension and Other Post-retirement Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>100220 - Disclosure - Employee Stock Ownership Plan</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureEmployeeStockOwnershipPlan</Role>
      <ShortName>Employee Stock Ownership Plan</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>100230 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>100240 - Disclosure - Fair Value of Assets and Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureFairValueOfAssetsAndLiabilities</Role>
      <ShortName>Fair Value of Assets and Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>100250 - Disclosure - Other Non-Interest Expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureOtherNonInterestExpenses</Role>
      <ShortName>Other Non-Interest Expenses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>100260 - Disclosure - Parent Company Condensed Financial Statements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureParentCompanyCondensedFinancialStatements</Role>
      <ShortName>Parent Company Condensed Financial Statements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>100270 - Disclosure - Selected Quarterly Financial Data (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnaudited</Role>
      <ShortName>Selected Quarterly Financial Data (Unaudited)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>100280 - Disclosure - Nature of Operations and Basis of Presentation (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureNatureOfOperationsAndBasisOfPresentationPolicies</Role>
      <ShortName>Nature of Operations and Basis of Presentation (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>100290 - Disclosure - Nature of Operations and Basis of Presentation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureNatureOfOperationsAndBasisOfPresentationTables</Role>
      <ShortName>Nature of Operations and Basis of Presentation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureNatureOfOperationsAndBasisOfPresentation</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>100300 - Disclosure - Acquisition (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureAcquisitionTables</Role>
      <ShortName>Acquisition (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureAcquisition</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>100310 - Disclosure - Securities Available for Sale (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSecuritiesAvailableForSaleTables</Role>
      <ShortName>Securities Available for Sale (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSecuritiesAvailableForSale</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>100320 - Disclosure - Loans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoansTables</Role>
      <ShortName>Loans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoans</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>100330 - Disclosure - Loan Servicing (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoanServicingTables</Role>
      <ShortName>Loan Servicing (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoanServicing</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>100340 - Disclosure - Premises and Equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePremisesAndEquipmentTables</Role>
      <ShortName>Premises and Equipment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePremisesAndEquipment</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>100350 - Disclosure - Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureDepositsTables</Role>
      <ShortName>Deposits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureDeposits</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>100360 - Disclosure - Borrowings (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureBorrowingsTables</Role>
      <ShortName>Borrowings (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureBorrowings</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>100370 - Disclosure - Income Taxes (Table)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureIncomeTaxesTable</Role>
      <ShortName>Income Taxes (Table)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureIncomeTaxes</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>100380 - Disclosure - Minimum Regulatory Capital Requirements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureMinimumRegulatoryCapitalRequirementsTables</Role>
      <ShortName>Minimum Regulatory Capital Requirements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureMinimumRegulatoryCapitalRequirements</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>100390 - Disclosure - Pension and Other Post-retirement Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePensionAndOtherPostRetirementPlansTables</Role>
      <ShortName>Pension and Other Post-retirement Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePensionAndOtherPostRetirementPlans</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>100400 - Disclosure - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureCommitmentsAndContingenciesTables</Role>
      <ShortName>Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureCommitmentsAndContingencies</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>100410 - Disclosure - Fair Value of Assets and Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureFairValueOfAssetsAndLiabilitiesTables</Role>
      <ShortName>Fair Value of Assets and Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureFairValueOfAssetsAndLiabilities</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>100420 - Disclosure - Other Non-Interest Expenses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureOtherNonInterestExpensesTables</Role>
      <ShortName>Other Non-Interest Expenses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureOtherNonInterestExpenses</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>100430 - Disclosure - Parent Company Condensed Financial Statements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureParentCompanyCondensedFinancialStatementsTables</Role>
      <ShortName>Parent Company Condensed Financial Statements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureParentCompanyCondensedFinancialStatements</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>100440 - Disclosure - Selected Quarterly Financial Data (Unaudited) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnauditedTables</Role>
      <ShortName>Selected Quarterly Financial Data (Unaudited) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnaudited</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>100450 - Disclosure - Conversion - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureConversionAdditionalInformationDetail</Role>
      <ShortName>Conversion - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>100460 - Disclosure - Nature of Operations and Basis of Presentation - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureNatureOfOperationsAndBasisOfPresentationAdditionalInformationDetail</Role>
      <ShortName>Nature of Operations and Basis of Presentation - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>100470 - Disclosure - Nature of Operations and Basis of Presentation - Components of Accumulated Other Comprehensive Income (Loss) and Related Tax Effects (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureNatureOfOperationsAndBasisOfPresentationComponentsOfAccumulatedOtherComprehensiveIncomeLossAndRelatedTaxEffectsDetail</Role>
      <ShortName>Nature of Operations and Basis of Presentation - Components of Accumulated Other Comprehensive Income (Loss) and Related Tax Effects (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>100480 - Disclosure - Acquisition - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureAcquisitionAdditionalInformationDetail</Role>
      <ShortName>Acquisition - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>100490 - Disclosure - Acquisition - Summary of Cash Consideration Paid, and Fair Values of Assets Acquired and Liabilities Assumed (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureAcquisitionSummaryOfCashConsiderationPaidAndFairValuesOfAssetsAcquiredAndLiabilitiesAssumedDetail</Role>
      <ShortName>Acquisition - Summary of Cash Consideration Paid, and Fair Values of Assets Acquired and Liabilities Assumed (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>100500 - Disclosure - Acquisition - Summary of Cash Consideration Paid, and Fair Values of Assets Acquired and Liabilities Assumed (Parenthetical) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureAcquisitionSummaryOfCashConsiderationPaidAndFairValuesOfAssetsAcquiredAndLiabilitiesAssumedParentheticalDetail</Role>
      <ShortName>Acquisition - Summary of Cash Consideration Paid, and Fair Values of Assets Acquired and Liabilities Assumed (Parenthetical) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>100510 - Disclosure - Acquisition - Summary of Unaudited Pro Forma Financial Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureAcquisitionSummaryOfUnauditedProFormaFinancialInformationDetail</Role>
      <ShortName>Acquisition - Summary of Unaudited Pro Forma Financial Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>100520 - Disclosure - Securities Available for Sale - Schedule of Amortized Cost and Fair Value of Securities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSecuritiesAvailableForSaleScheduleOfAmortizedCostAndFairValueOfSecuritiesDetail</Role>
      <ShortName>Securities Available for Sale - Schedule of Amortized Cost and Fair Value of Securities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>100530 - Disclosure - Securities Available for Sale - Investments Classified by Contractual Maturity Date (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSecuritiesAvailableForSaleInvestmentsClassifiedByContractualMaturityDateDetail</Role>
      <ShortName>Securities Available for Sale - Investments Classified by Contractual Maturity Date (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>100540 - Disclosure - Securities Available for Sale - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSecuritiesAvailableForSaleAdditionalInformationDetail</Role>
      <ShortName>Securities Available for Sale - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>100550 - Disclosure - Securities Available for Sale - Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSecuritiesAvailableForSaleAvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValueDetail</Role>
      <ShortName>Securities Available for Sale - Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>100560 - Disclosure - Loans - Summary of Loan Portfolio (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoansSummaryOfLoanPortfolioDetail</Role>
      <ShortName>Loans - Summary of Loan Portfolio (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>100570 - Disclosure - Loans - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoansAdditionalInformationDetail</Role>
      <ShortName>Loans - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>100580 - Disclosure - Loans - Summary of Changes in Loans to Directors, Executive Officers and Affiliates (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoansSummaryOfChangesInLoansToDirectorsExecutiveOfficersAndAffiliatesDetail</Role>
      <ShortName>Loans - Summary of Changes in Loans to Directors, Executive Officers and Affiliates (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>100590 - Disclosure - Loans - Summary of Activity in the Allowance for Loan Losses by Loan Category (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoansSummaryOfActivityInAllowanceForLoanLossesByLoanCategoryDetail</Role>
      <ShortName>Loans - Summary of Activity in the Allowance for Loan Losses by Loan Category (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>100600 - Disclosure - Loans - Summary of Additional Information Pertaining to the Allowance for Loan Losses (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoansSummaryOfAdditionalInformationPertainingToAllowanceForLoanLossesDetail</Role>
      <ShortName>Loans - Summary of Additional Information Pertaining to the Allowance for Loan Losses (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>100610 - Disclosure - Loans - Schedule of Past Due and Non-Accrual Loans by Loan Category (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoansScheduleOfPastDueAndNonAccrualLoansByLoanCategoryDetail</Role>
      <ShortName>Loans - Schedule of Past Due and Non-Accrual Loans by Loan Category (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>100620 - Disclosure - Loans - Summary of Impaired Loans (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoansSummaryOfImpairedLoansDetail</Role>
      <ShortName>Loans - Summary of Impaired Loans (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>100630 - Disclosure - Loans - Summary of Information Related to Average Balances of Impaired Loans and Interest Income Recognized (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoansSummaryOfInformationRelatedToAverageBalancesOfImpairedLoansAndInterestIncomeRecognizedDetail</Role>
      <ShortName>Loans - Summary of Information Related to Average Balances of Impaired Loans and Interest Income Recognized (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>100640 - Disclosure - Loans - Summary of Company's Loans by Risk Rating (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoansSummaryOfCompanySLoansByRiskRatingDetail</Role>
      <ShortName>Loans - Summary of Company's Loans by Risk Rating (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>100650 - Disclosure - Loan Servicing - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoanServicingAdditionalInformationDetail</Role>
      <ShortName>Loan Servicing - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>100660 - Disclosure - Loan Servicing - Summary of Activity In Company's Mortgage Servicing Rights (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureLoanServicingSummaryOfActivityInCompanySMortgageServicingRightsDetail</Role>
      <ShortName>Loan Servicing - Summary of Activity In Company's Mortgage Servicing Rights (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>100670 - Disclosure - Premises and Equipment - Summary of Cost and Accumulated Depreciation and Amortization of Premises and Equipment (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePremisesAndEquipmentSummaryOfCostAndAccumulatedDepreciationAndAmortizationOfPremisesAndEquipmentDetail</Role>
      <ShortName>Premises and Equipment - Summary of Cost and Accumulated Depreciation and Amortization of Premises and Equipment (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>100680 - Disclosure - Premises and Equipment - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePremisesAndEquipmentAdditionalInformationDetail</Role>
      <ShortName>Premises and Equipment - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>100690 - Disclosure - Deposits - Summary of Deposit Balances by Type (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureDepositsSummaryOfDepositBalancesByTypeDetail</Role>
      <ShortName>Deposits - Summary of Deposit Balances by Type (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>100700 - Disclosure - Deposits - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureDepositsAdditionalInformationDetail</Role>
      <ShortName>Deposits - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>100710 - Disclosure - Deposits - Summary of Term Certificates by Maturity (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureDepositsSummaryOfTermCertificatesByMaturityDetail</Role>
      <ShortName>Deposits - Summary of Term Certificates by Maturity (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>100720 - Disclosure - Borrowings - Summary of Borrowings from FHLBB (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureBorrowingsSummaryOfBorrowingsFromFHLBBDetail</Role>
      <ShortName>Borrowings - Summary of Borrowings from FHLBB (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>100730 - Disclosure - Borrowings - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureBorrowingsAdditionalInformationDetail</Role>
      <ShortName>Borrowings - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>100740 - Disclosure - Income Taxes - Allocation of Federal and State Income Taxes Between Current and Deferred Portions (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureIncomeTaxesAllocationOfFederalAndStateIncomeTaxesBetweenCurrentAndDeferredPortionsDetail</Role>
      <ShortName>Income Taxes - Allocation of Federal and State Income Taxes Between Current and Deferred Portions (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>100750 - Disclosure - Income Taxes - Summary of Differences Between Statutory Federal Income Tax Expense (Benefit) and Actual Tax Expense (Benefit) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureIncomeTaxesSummaryOfDifferencesBetweenStatutoryFederalIncomeTaxExpenseBenefitAndActualTaxExpenseBenefitDetail</Role>
      <ShortName>Income Taxes - Summary of Differences Between Statutory Federal Income Tax Expense (Benefit) and Actual Tax Expense (Benefit) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>100760 - Disclosure - Income Taxes - Summary of Differences Between Statutory Federal Income Tax Expense (Benefit) and Actual Tax Expense (Benefit) (Parenthetical) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureIncomeTaxesSummaryOfDifferencesBetweenStatutoryFederalIncomeTaxExpenseBenefitAndActualTaxExpenseBenefitParentheticalDetail</Role>
      <ShortName>Income Taxes - Summary of Differences Between Statutory Federal Income Tax Expense (Benefit) and Actual Tax Expense (Benefit) (Parenthetical) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>100770 - Disclosure - Income Taxes - Summary of Components of Net Deferred Tax Asset (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureIncomeTaxesSummaryOfComponentsOfNetDeferredTaxAssetDetail</Role>
      <ShortName>Income Taxes - Summary of Components of Net Deferred Tax Asset (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>100780 - Disclosure - Income Taxes - Summary of Deferred Tax Asset (Liabilities) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureIncomeTaxesSummaryOfDeferredTaxAssetLiabilitiesDetail</Role>
      <ShortName>Income Taxes - Summary of Deferred Tax Asset (Liabilities) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>100790 - Disclosure - Income Taxes - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureIncomeTaxesAdditionalInformationDetail</Role>
      <ShortName>Income Taxes - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>100800 - Disclosure - Income Taxes - Summary of Change in Net Deferred Tax Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureIncomeTaxesSummaryOfChangeInNetDeferredTaxAssetsDetail</Role>
      <ShortName>Income Taxes - Summary of Change in Net Deferred Tax Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>100810 - Disclosure - On-Balance Sheet Derivative Instruments and Hedging Activities - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureOnBalanceSheetDerivativeInstrumentsAndHedgingActivitiesAdditionalInformationDetail</Role>
      <ShortName>On-Balance Sheet Derivative Instruments and Hedging Activities - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>100820 - Disclosure - Minimum Regulatory Capital Requirements - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureMinimumRegulatoryCapitalRequirementsAdditionalInformationDetail</Role>
      <ShortName>Minimum Regulatory Capital Requirements - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>100830 - Disclosure - Minimum Regulatory Capital Requirements - Summary of Actual and Minimum Capital Amounts and Ratios (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureMinimumRegulatoryCapitalRequirementsSummaryOfActualAndMinimumCapitalAmountsAndRatiosDetail</Role>
      <ShortName>Minimum Regulatory Capital Requirements - Summary of Actual and Minimum Capital Amounts and Ratios (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>100840 - Disclosure - Pension and Other Post-Retirement Plans - Summary of Information Pertaining to Activity in Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePensionAndOtherPostRetirementPlansSummaryOfInformationPertainingToActivityInPlanDetails</Role>
      <ShortName>Pension and Other Post-Retirement Plans - Summary of Information Pertaining to Activity in Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>100850 - Disclosure - Pension and Other Post-Retirement Plans - Summary of Net Periodic Pension Cost Included in Salaries and Employee Benefits Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePensionAndOtherPostRetirementPlansSummaryOfNetPeriodicPensionCostIncludedInSalariesAndEmployeeBenefitsExpenseDetails</Role>
      <ShortName>Pension and Other Post-Retirement Plans - Summary of Net Periodic Pension Cost Included in Salaries and Employee Benefits Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>100860 - Disclosure - Pension and Other Post-Retirement Plans - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePensionAndOtherPostRetirementPlansAdditionalInformationDetail</Role>
      <ShortName>Pension and Other Post-Retirement Plans - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>100870 - Disclosure - Pension and Other Post-Retirement Plans - Summary of Assumptions Used to Determine Benefit Obligation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePensionAndOtherPostRetirementPlansSummaryOfAssumptionsUsedToDetermineBenefitObligationDetails</Role>
      <ShortName>Pension and Other Post-Retirement Plans - Summary of Assumptions Used to Determine Benefit Obligation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>100880 - Disclosure - Pension and Other Post-Retirement Plans - Summary of Net Periodic Pension Cost Included in Other Non-Interest Expenses Attributable to Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePensionAndOtherPostRetirementPlansSummaryOfNetPeriodicPensionCostIncludedInOtherNonInterestExpensesAttributableToPlanDetails</Role>
      <ShortName>Pension and Other Post-Retirement Plans - Summary of Net Periodic Pension Cost Included in Other Non-Interest Expenses Attributable to Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>100890 - Disclosure - Pension and Other Post-Retirement Plans - Summary of Assumptions Used to Determine Net Periodic Pension Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePensionAndOtherPostRetirementPlansSummaryOfAssumptionsUsedToDetermineNetPeriodicPensionCostDetails</Role>
      <ShortName>Pension and Other Post-Retirement Plans - Summary of Assumptions Used to Determine Net Periodic Pension Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>100900 - Disclosure - Pension and Other Post-Retirement Plans - Summary of Estimated Future Benefit Payments Expected Future Services (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosurePensionAndOtherPostRetirementPlansSummaryOfEstimatedFutureBenefitPaymentsExpectedFutureServicesDetails</Role>
      <ShortName>Pension and Other Post-Retirement Plans - Summary of Estimated Future Benefit Payments Expected Future Services (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>100910 - Disclosure - Employee Stock Ownership Plan - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureEmployeeStockOwnershipPlanAdditionalInformationDetail</Role>
      <ShortName>Employee Stock Ownership Plan - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>100920 - Disclosure - Commitments and Contingencies - Schedule of Future Minimum Rent Commitments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureCommitmentsAndContingenciesScheduleOfFutureMinimumRentCommitmentsDetail</Role>
      <ShortName>Commitments and Contingencies - Schedule of Future Minimum Rent Commitments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>100930 - Disclosure - Commitments and Contingencies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetail</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>100940 - Disclosure - Commitments and Contingencies - Summary of Financial Instruments Outstanding Contract Amounts Represent Credit Risk (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureCommitmentsAndContingenciesSummaryOfFinancialInstrumentsOutstandingContractAmountsRepresentCreditRiskDetail</Role>
      <ShortName>Commitments and Contingencies - Summary of Financial Instruments Outstanding Contract Amounts Represent Credit Risk (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>100950 - Disclosure - Fair Value of Assets and Liabilities - Additional information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureFairValueOfAssetsAndLiabilitiesAdditionalInformationDetail</Role>
      <ShortName>Fair Value of Assets and Liabilities - Additional information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>100960 - Disclosure - Fair Value of Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureFairValueOfAssetsAndLiabilitiesScheduleOfAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetail</Role>
      <ShortName>Fair Value of Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>100970 - Disclosure - Fair Value of Assets and Liabilities - Schedule of Assets Measured at Fair Value on a Non-Recurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureFairValueOfAssetsAndLiabilitiesScheduleOfAssetsMeasuredAtFairValueOnNonRecurringBasisDetail</Role>
      <ShortName>Fair Value of Assets and Liabilities - Schedule of Assets Measured at Fair Value on a Non-Recurring Basis (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>100980 - Disclosure - Fair Value of Assets and Liabilities - Summary of Carrying Values, Estimated Fair Values and Placement in Fair Value Hierarchy of Company's Financial Instruments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureFairValueOfAssetsAndLiabilitiesSummaryOfCarryingValuesEstimatedFairValuesAndPlacementInFairValueHierarchyOfCompanySFinancialInstrumentsDetail</Role>
      <ShortName>Fair Value of Assets and Liabilities - Summary of Carrying Values, Estimated Fair Values and Placement in Fair Value Hierarchy of Company's Financial Instruments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>100990 - Disclosure - Other Non-Interest Expenses - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureOtherNonInterestExpensesAdditionalInformationDetail</Role>
      <ShortName>Other Non-Interest Expenses - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>101000 - Disclosure - Other Non-Interest Expenses - Schedule of Company's Total Interest and Non-Interest Income Included in Other Non-Interest Expenses (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureOtherNonInterestExpensesScheduleOfCompanySTotalInterestAndNonInterestIncomeIncludedInOtherNonInterestExpensesDetail</Role>
      <ShortName>Other Non-Interest Expenses - Schedule of Company's Total Interest and Non-Interest Income Included in Other Non-Interest Expenses (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>100</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>101010 - Disclosure - Parent Company Condensed Financial Statements - Balance Sheets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureParentCompanyCondensedFinancialStatementsBalanceSheetsDetail</Role>
      <ShortName>Parent Company Condensed Financial Statements - Balance Sheets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>101020 - Disclosure - Parent Company Condensed Financial Statements - Statements of Operations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureParentCompanyCondensedFinancialStatementsStatementsOfOperationsDetail</Role>
      <ShortName>Parent Company Condensed Financial Statements - Statements of Operations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>101030 - Disclosure - Parent Company Condensed Financial Statements - Statements of Cash Flows (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureParentCompanyCondensedFinancialStatementsStatementsOfCashFlowsDetail</Role>
      <ShortName>Parent Company Condensed Financial Statements - Statements of Cash Flows (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>103</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>101040 - Disclosure - Selected Quarterly Financial Data (Unaudited) - Summary of Selected Quarterly Financial Data (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnauditedSummaryOfSelectedQuarterlyFinancialDataDetail</Role>
      <ShortName>Selected Quarterly Financial Data (Unaudited) - Summary of Selected Quarterly Financial Data (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnauditedTables</ParentRole>
      <Position>104</Position>
    </Report>
    <Report instance="rndb-20161231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>101050 - Disclosure - Selected Quarterly Financial Data (Unaudited) - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnauditedAdditionalInformationDetail</Role>
      <ShortName>Selected Quarterly Financial Data (Unaudited) - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.randolphsavings.com/20161231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnauditedTables</ParentRole>
      <Position>105</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>rndb-20161231.xml</File>
    <File>rndb-20161231.xsd</File>
    <File>rndb-20161231_cal.xml</File>
    <File>rndb-20161231_def.xml</File>
    <File>rndb-20161231_lab.xml</File>
    <File>rndb-20161231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies/>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
