<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.21.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>312</ContextCount>
  <ElementCount>314</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>66</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>7</UnitCount>
  <MyReports>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>1001 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1002 - Statement - CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1003 - Statement - CONSOLIDATED BALANCE SHEETS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1004 - Statement - CONSOLIDATED STATEMENTS OF INCOME (LOSS) AND COMPREHENSIVE INCOME (LOSS)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/ConsolidatedStatementsOfIncomeLossAndComprehensiveIncomeLoss</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF INCOME (LOSS) AND COMPREHENSIVE INCOME (LOSS)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1005 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/ConsolidatedStatementsOfChangesInStockholdersEquity</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1006 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/ConsolidatedStatementsOfChangesInStockholdersEquityParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1007 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/ConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>1008 - Disclosure - Summary of significant accounting policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/SummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of significant accounting policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>1009 - Disclosure - Other comprehensive income (loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/OtherComprehensiveIncomeLoss</Role>
      <ShortName>Other comprehensive income (loss)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>1010 - Disclosure - Earnings per share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/EarningsPerShare</Role>
      <ShortName>Earnings per share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>1011 - Disclosure - Investment securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/InvestmentSecurities</Role>
      <ShortName>Investment securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>1012 - Disclosure - Loans, net, and allowance for loan losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/LoansNetAndAllowanceForLoanLosses</Role>
      <ShortName>Loans, net, and allowance for loan losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>1013 - Disclosure - Other assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/OtherAssets</Role>
      <ShortName>Other assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>1014 - Disclosure - Fair value estimates</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/FairValueEstimates</Role>
      <ShortName>Fair value estimates</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>1015 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/SubsequentEvents</Role>
      <ShortName>Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>1016 - Disclosure - Summary of significant accounting policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/SummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of significant accounting policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>1017 - Disclosure - Other comprehensive income (loss) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/OtherComprehensiveIncomeLossTables</Role>
      <ShortName>Other comprehensive income (loss) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fnbmarysville.com/role/OtherComprehensiveIncomeLoss</ParentRole>
      <Position>17</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>1018 - Disclosure - Earnings per share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/EarningsPerShareTables</Role>
      <ShortName>Earnings per share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fnbmarysville.com/role/EarningsPerShare</ParentRole>
      <Position>18</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>1019 - Disclosure - Investment securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/InvestmentSecuritiesTables</Role>
      <ShortName>Investment securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fnbmarysville.com/role/InvestmentSecurities</ParentRole>
      <Position>19</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>1020 - Disclosure - Loans, net, and allowance for loan losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/LoansNetAndAllowanceForLoanLossesTables</Role>
      <ShortName>Loans, net, and allowance for loan losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fnbmarysville.com/role/LoansNetAndAllowanceForLoanLosses</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>1021 - Disclosure - Other assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/OtherAssetsTables</Role>
      <ShortName>Other assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fnbmarysville.com/role/OtherAssets</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>1022 - Disclosure - Fair value estimates (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/FairValueEstimatesTables</Role>
      <ShortName>Fair value estimates (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fnbmarysville.com/role/FairValueEstimates</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>1023 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/SummaryOfSignificantAccountingPoliciesAdditionalInformationDetail</Role>
      <ShortName>Summary of Significant Accounting Policies - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>1024 - Disclosure - Other comprehensive income (loss) - Summary of Components of Accumulated Other Comprehensive Income (Loss) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/OtherComprehensiveIncomeLossSummaryOfComponentsOfAccumulatedOtherComprehensiveIncomeLossDetail</Role>
      <ShortName>Other comprehensive income (loss) - Summary of Components of Accumulated Other Comprehensive Income (Loss) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fnbmarysville.com/role/OtherComprehensiveIncomeLossTables</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>1025 - Disclosure - Other comprehensive income (loss) - Schedule of Other Comprehensive Income (Loss) and Related Tax Effects (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/OtherComprehensiveIncomeLossScheduleOfOtherComprehensiveIncomeLossAndRelatedTaxEffectsDetail</Role>
      <ShortName>Other comprehensive income (loss) - Schedule of Other Comprehensive Income (Loss) and Related Tax Effects (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fnbmarysville.com/role/OtherComprehensiveIncomeLossTables</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>1026 - Disclosure - Earnings per share - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/EarningsPerShareAdditionalInformationDetail</Role>
      <ShortName>Earnings per share - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>1027 - Disclosure - Earnings per share - Computation of Earnings per Share (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/EarningsPerShareComputationOfEarningsPerShareDetail</Role>
      <ShortName>Earnings per share - Computation of Earnings per Share (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>1028 - Disclosure - Investment Securities - Schedule of Amortized Cost and Fair Value of Investment Securities Available-for-Sale Aggregated by Investment Category (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/InvestmentSecuritiesScheduleOfAmortizedCostAndFairValueOfInvestmentSecuritiesAvailableForSaleAggregatedByInvestmentCategoryDetail</Role>
      <ShortName>Investment Securities - Schedule of Amortized Cost and Fair Value of Investment Securities Available-for-Sale Aggregated by Investment Category (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>1029 - Disclosure - Investment Securities - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/InvestmentSecuritiesAdditionalInformationDetail</Role>
      <ShortName>Investment Securities - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>1030 - Disclosure - Investment Securities - Schedule of Debt Securities Classified Available-for-Sale Maturity Distribution of Fair Value (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/InvestmentSecuritiesScheduleOfDebtSecuritiesClassifiedAvailableForSaleMaturityDistributionOfFairValueDetail</Role>
      <ShortName>Investment Securities - Schedule of Debt Securities Classified Available-for-Sale Maturity Distribution of Fair Value (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>1031 - Disclosure - Investment Securities Available-for-Sale - Schedule of Fair Value Gross Unrealized Losses of Investment Securities Unrealized Losses (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/InvestmentSecuritiesAvailableForSaleScheduleOfFairValueGrossUnrealizedLossesOfInvestmentSecuritiesUnrealizedLossesDetail</Role>
      <ShortName>Investment Securities Available-for-Sale - Schedule of Fair Value Gross Unrealized Losses of Investment Securities Unrealized Losses (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>1032 - Disclosure - Loans, Net, and Allowance for Loan Losses - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/LoansNetAndAllowanceForLoanLossesAdditionalInformationDetail</Role>
      <ShortName>Loans, Net, and Allowance for Loan Losses - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>1033 - Disclosure - Loans, Net, and Allowance for Loan Losses - Schedule of Loans Outstanding (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/LoansNetAndAllowanceForLoanLossesScheduleOfLoansOutstandingDetail</Role>
      <ShortName>Loans, Net, and Allowance for Loan Losses - Schedule of Loans Outstanding (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>1034 - Disclosure - Loans, Net, and Allowance for Loan Losses - Schedule of Allowance for Loan Losses Account by Major Classification of Loan (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/LoansNetAndAllowanceForLoanLossesScheduleOfAllowanceForLoanLossesAccountByMajorClassificationOfLoanDetail</Role>
      <ShortName>Loans, Net, and Allowance for Loan Losses - Schedule of Allowance for Loan Losses Account by Major Classification of Loan (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>1035 - Disclosure - Loans, Net, and Allowance for Loan Losses - Summary of Major Classification of Loans Summarized by Aggregate Pass Rating (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/LoansNetAndAllowanceForLoanLossesSummaryOfMajorClassificationOfLoansSummarizedByAggregatePassRatingDetail</Role>
      <ShortName>Loans, Net, and Allowance for Loan Losses - Summary of Major Classification of Loans Summarized by Aggregate Pass Rating (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>1036 - Disclosure - Loans, Net, and Allowance for Loan Losses - Summary of Classes of Loan Portfolio Summarized by Aging Categories of Performing Loans and Nonaccrual Loans (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/LoansNetAndAllowanceForLoanLossesSummaryOfClassesOfLoanPortfolioSummarizedByAgingCategoriesOfPerformingLoansAndNonaccrualLoansDetail</Role>
      <ShortName>Loans, Net, and Allowance for Loan Losses - Summary of Classes of Loan Portfolio Summarized by Aging Categories of Performing Loans and Nonaccrual Loans (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>1037 - Disclosure - Loans, Net, and Allowance for Loan Losses - Schedule of Information Concerning Impaired Loans (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/LoansNetAndAllowanceForLoanLossesScheduleOfInformationConcerningImpairedLoansDetail</Role>
      <ShortName>Loans, Net, and Allowance for Loan Losses - Schedule of Information Concerning Impaired Loans (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>1038 - Disclosure - Loans, net, and allowance for loan losses - Summary of Distribution of Off-Balance Sheet Commitments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/LoansNetAndAllowanceForLoanLossesSummaryOfDistributionOfOffBalanceSheetCommitmentsDetail</Role>
      <ShortName>Loans, net, and allowance for loan losses - Summary of Distribution of Off-Balance Sheet Commitments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>1039 - Disclosure - Other Assets - Components of Other Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/OtherAssetsComponentsOfOtherAssetsDetail</Role>
      <ShortName>Other Assets - Components of Other Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>1040 - Disclosure - Fair value estimates - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/FairValueEstimatesFinancialAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetail</Role>
      <ShortName>Fair value estimates - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>1041 - Disclosure - Fair value estimates - Summary of Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/FairValueEstimatesSummaryOfAssetsAndLiabilitiesMeasuredAtFairValueOnANonrecurringBasisDetail</Role>
      <ShortName>Fair value estimates - Summary of Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>1042 - Disclosure - Fair value estimates- Additional Quantitative Information about Assets Measured at Fair Value on Nonrecurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/FairValueEstimatesAdditionalQuantitativeInformationAboutAssetsMeasuredAtFairValueOnNonrecurringBasisDetail</Role>
      <ShortName>Fair value estimates- Additional Quantitative Information about Assets Measured at Fair Value on Nonrecurring Basis (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="d185094d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>1043 - Disclosure - Fair value estimates- Carrying and Fair Values of Riverview's Financial Instruments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fnbmarysville.com/role/FairValueEstimatesCarryingAndFairValuesOfRiverviewsFinancialInstrumentsDetail</Role>
      <ShortName>Fair value estimates- Carrying and Fair Values of Riverview's Financial Instruments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" original="d185094d10q.htm">d185094d10q.htm</File>
    <File>d185094dex311.htm</File>
    <File>d185094dex312.htm</File>
    <File>d185094dex321.htm</File>
    <File>d185094dex322.htm</File>
    <File>rive-20210630.xsd</File>
    <File>rive-20210630_cal.xml</File>
    <File>rive-20210630_def.xml</File>
    <File>rive-20210630_lab.xml</File>
    <File>rive-20210630_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/srt/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2020-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
