<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>536</ContextCount>
  <ElementCount>450</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>107</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>9</UnitCount>
  <MyReports>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/Cover</Role>
      <ShortName>Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>9952151 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>9952152 - Statement - Consolidated Balance Sheets (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/ConsolidatedBalanceSheetsParentheticals</Role>
      <ShortName>Consolidated Balance Sheets (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>9952153 - Statement - Consolidated Statements of Income and Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/ConsolidatedStatementsofIncomeandComprehensiveIncome</Role>
      <ShortName>Consolidated Statements of Income and Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>9952154 - Statement - Consolidated Statements of Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/ConsolidatedStatementsofStockholdersEquity</Role>
      <ShortName>Consolidated Statements of Stockholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>9952155 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/ConsolidatedStatementsofCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>9952156 - Disclosure - Organization and Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/OrganizationandBasisofPresentation</Role>
      <ShortName>Organization and Basis of Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>9952157 - Disclosure - Acquisition of First Foundation Inc.</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/AcquisitionofFirstFoundationInc</Role>
      <ShortName>Acquisition of First Foundation Inc.</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>9952158 - Disclosure - Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/Securities</Role>
      <ShortName>Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>9952159 - Disclosure - Loans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/Loans</Role>
      <ShortName>Loans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>9952160 - Disclosure - Mortgage Servicing Rights</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/MortgageServicingRights</Role>
      <ShortName>Mortgage Servicing Rights</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>9952161 - Disclosure - Derivative Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/DerivativeFinancialInstruments</Role>
      <ShortName>Derivative Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>9952162 - Disclosure - Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/Deposits</Role>
      <ShortName>Deposits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>9952163 - Disclosure - Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/Debt</Role>
      <ShortName>Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>9952164 - Disclosure - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/EarningsPerShare</Role>
      <ShortName>Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>9952165 - Disclosure - Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/StockholdersEquity</Role>
      <ShortName>Stockholders' Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>9952166 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>9952167 - Disclosure - Regulatory Capital Matters</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/RegulatoryCapitalMatters</Role>
      <ShortName>Regulatory Capital Matters</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>9952168 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/FairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>9952169 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/SegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>9952170 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/CommitmentsandContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>9952171 - Disclosure - Lease Commitments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LeaseCommitments</Role>
      <ShortName>Lease Commitments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>9952172 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/SubsequentEvents</Role>
      <ShortName>Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Insider Trading Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements</Role>
      <ShortName>Insider Trading Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9955511 - Disclosure - Organization and Basis of Presentation (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/OrganizationandBasisofPresentationPolicies</Role>
      <ShortName>Organization and Basis of Presentation (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9955512 - Disclosure - Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/SecuritiesTables</Role>
      <ShortName>Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/Securities</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9955513 - Disclosure - Loans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LoansTables</Role>
      <ShortName>Loans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/Loans</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9955514 - Disclosure - Mortgage Servicing Rights (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/MortgageServicingRightsTables</Role>
      <ShortName>Mortgage Servicing Rights (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/MortgageServicingRights</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9955515 - Disclosure - Derivative Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/DerivativeFinancialInstrumentsTables</Role>
      <ShortName>Derivative Financial Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/DerivativeFinancialInstruments</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9955516 - Disclosure - Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/DepositsTables</Role>
      <ShortName>Deposits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/Deposits</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>9955517 - Disclosure - Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/DebtTables</Role>
      <ShortName>Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/Debt</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>9955518 - Disclosure - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/EarningsPerShareTables</Role>
      <ShortName>Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/EarningsPerShare</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>9955519 - Disclosure - Stockholders' Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/StockholdersEquityTables</Role>
      <ShortName>Stockholders' Equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/StockholdersEquity</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9955520 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/IncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/IncomeTaxes</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9955521 - Disclosure - Regulatory Capital Matters (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/RegulatoryCapitalMattersTables</Role>
      <ShortName>Regulatory Capital Matters (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/RegulatoryCapitalMatters</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9955522 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/FairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/FairValueMeasurements</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9955523 - Disclosure - Segment Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/SegmentInformationTables</Role>
      <ShortName>Segment Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/SegmentInformation</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9955524 - Disclosure - Lease Commitments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LeaseCommitmentsTables</Role>
      <ShortName>Lease Commitments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/LeaseCommitments</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9955525 - Disclosure - Acquisition of First Foundation Inc. (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/AcquisitionofFirstFoundationIncDetails</Role>
      <ShortName>Acquisition of First Foundation Inc. (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/AcquisitionofFirstFoundationInc</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9955526 - Disclosure - Securities - Schedule of Securities by Type (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/SecuritiesScheduleofSecuritiesbyTypeDetails</Role>
      <ShortName>Securities - Schedule of Securities by Type (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9955527 - Disclosure - Securities - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/SecuritiesNarrativeDetails</Role>
      <ShortName>Securities - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9955528 - Disclosure - Securities - Schedule of Fair Value and Unrealized Losses on Debt Securities in Continuous Unrealized Loss Position (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/SecuritiesScheduleofFairValueandUnrealizedLossesonDebtSecuritiesinContinuousUnrealizedLossPositionDetails</Role>
      <ShortName>Securities - Schedule of Fair Value and Unrealized Losses on Debt Securities in Continuous Unrealized Loss Position (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9955529 - Disclosure - Securities - Schedule of Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/SecuritiesScheduleofAmortizedCostandEstimatedFairValueofDebtSecuritiesbyContractualMaturityDetails</Role>
      <ShortName>Securities - Schedule of Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9955530 - Disclosure - Loans - Schedule of Loans Held for Investment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LoansScheduleofLoansHeldforInvestmentDetails</Role>
      <ShortName>Loans - Schedule of Loans Held for Investment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9955531 - Disclosure - Loans - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LoansNarrativeDetails</Role>
      <ShortName>Loans - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9955532 - Disclosure - Loans - Schedule of Allowance for Credit Losses by Portfolio Type (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LoansScheduleofAllowanceforCreditLossesbyPortfolioTypeDetails</Role>
      <ShortName>Loans - Schedule of Allowance for Credit Losses by Portfolio Type (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9955533 - Disclosure - Loans - Schedule of Aging of Loan Portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LoansScheduleofAgingofLoanPortfolioDetails</Role>
      <ShortName>Loans - Schedule of Aging of Loan Portfolio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9955534 - Disclosure - Loans - Schedule of Amortized Costs by Segment of Loans by Risk Category and Origination Date (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LoansScheduleofAmortizedCostsbySegmentofLoansbyRiskCategoryandOriginationDateDetails</Role>
      <ShortName>Loans - Schedule of Amortized Costs by Segment of Loans by Risk Category and Origination Date (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9955535 - Disclosure - Loans - Schedule of Collateral Dependent Loans, Individually Evaluated for Allowance for Credit Loss (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LoansScheduleofCollateralDependentLoansIndividuallyEvaluatedforAllowanceforCreditLossDetails</Role>
      <ShortName>Loans - Schedule of Collateral Dependent Loans, Individually Evaluated for Allowance for Credit Loss (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9955536 - Disclosure - Loans - Schedule of Loan Modifications (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LoansScheduleofLoanModificationsDetails</Role>
      <ShortName>Loans - Schedule of Loan Modifications (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9955537 - Disclosure - Loans - Schedule of Performance Loan Modifications (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LoansScheduleofPerformanceLoanModificationsDetails</Role>
      <ShortName>Loans - Schedule of Performance Loan Modifications (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9955538 - Disclosure - Mortgage Servicing Rights - Schedule of Unpaid Principal Loan Balance of Servicing Portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/MortgageServicingRightsScheduleofUnpaidPrincipalLoanBalanceofServicingPortfolioDetails</Role>
      <ShortName>Mortgage Servicing Rights - Schedule of Unpaid Principal Loan Balance of Servicing Portfolio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9955539 - Disclosure - Mortgage Servicing Rights - Schedule of Mortgage Servicing Rights at Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/MortgageServicingRightsScheduleofMortgageServicingRightsatFairValueDetails</Role>
      <ShortName>Mortgage Servicing Rights - Schedule of Mortgage Servicing Rights at Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9955540 - Disclosure - Mortgage Servicing Rights - Schedule of Weighted-Average Key Assumptions to Estimate Fair Value of Mortgage Servicing Rights (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/MortgageServicingRightsScheduleofWeightedAverageKeyAssumptionstoEstimateFairValueofMortgageServicingRightsDetails</Role>
      <ShortName>Mortgage Servicing Rights - Schedule of Weighted-Average Key Assumptions to Estimate Fair Value of Mortgage Servicing Rights (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9955541 - Disclosure - Mortgage Servicing Rights - Schedule of Servicing and Ancillary Fees from Mortgage Servicing Portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/MortgageServicingRightsScheduleofServicingandAncillaryFeesfromMortgageServicingPortfolioDetails</Role>
      <ShortName>Mortgage Servicing Rights - Schedule of Servicing and Ancillary Fees from Mortgage Servicing Portfolio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>9955542 - Disclosure - Derivative Financial Instruments - Schedule of The Components Of Derivative Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/DerivativeFinancialInstrumentsScheduleofTheComponentsOfDerivativeFinancialInstrumentsDetails</Role>
      <ShortName>Derivative Financial Instruments - Schedule of The Components Of Derivative Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>9955543 - Disclosure - Derivative Financial Instruments - Schedule of Gains And Losses On Banking Derivatives Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/DerivativeFinancialInstrumentsScheduleofGainsAndLossesOnBankingDerivativesAssetsandLiabilitiesDetails</Role>
      <ShortName>Derivative Financial Instruments - Schedule of Gains And Losses On Banking Derivatives Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>9955544 - Disclosure - Derivative Financial Instruments - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/DerivativeFinancialInstrumentsNarrativeDetails</Role>
      <ShortName>Derivative Financial Instruments - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>9955545 - Disclosure - Derivative Financial Instruments- Schedule of Components of Our Mortgage Banking Derivative Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/DerivativeFinancialInstrumentsScheduleofComponentsofOurMortgageBankingDerivativeFinancialInstrumentsDetails</Role>
      <ShortName>Derivative Financial Instruments- Schedule of Components of Our Mortgage Banking Derivative Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>9955546 - Disclosure - Deposits - Schedule of Composition of Deposits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/DepositsScheduleofCompositionofDepositsDetails</Role>
      <ShortName>Deposits - Schedule of Composition of Deposits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>9955547 - Disclosure - Deposits - Schedule of Interest Expense Incurred on Deposits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/DepositsScheduleofInterestExpenseIncurredonDepositsDetails</Role>
      <ShortName>Deposits - Schedule of Interest Expense Incurred on Deposits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>9955548 - Disclosure - Deposits - Schedule of Remaining Maturity on Certificate of Deposit Accounts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/DepositsScheduleofRemainingMaturityonCertificateofDepositAccountsDetails</Role>
      <ShortName>Deposits - Schedule of Remaining Maturity on Certificate of Deposit Accounts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>9955549 - Disclosure - Debt - Schedule of FHLB Advances and Other Borrowings Outstanding (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/DebtScheduleofFHLBAdvancesandOtherBorrowingsOutstandingDetails</Role>
      <ShortName>Debt - Schedule of FHLB Advances and Other Borrowings Outstanding (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>9955550 - Disclosure - Debt - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/DebtNarrativeDetails</Role>
      <ShortName>Debt - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>9955551 - Disclosure - Earnings Per Share - Schedule of Computation of Basic and Diluted Earnings Per Share of Common Stock (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/EarningsPerShareScheduleofComputationofBasicandDilutedEarningsPerShareofCommonStockDetails</Role>
      <ShortName>Earnings Per Share - Schedule of Computation of Basic and Diluted Earnings Per Share of Common Stock (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>9955552 - Disclosure - Earnings Per Share - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/EarningsPerShareNarrativeDetails</Role>
      <ShortName>Earnings Per Share - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>9955553 - Disclosure - Stockholders' Equity - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/StockholdersEquityNarrativeDetails</Role>
      <ShortName>Stockholders' Equity - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>9955554 - Disclosure - Stockholders' Equity - Schedule of Dividends Payable (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/StockholdersEquityScheduleofDividendsPayableDetails</Role>
      <ShortName>Stockholders' Equity - Schedule of Dividends Payable (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>9955555 - Disclosure - Stockholders' Equity - Schedule of Stock Option Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/StockholdersEquityScheduleofStockOptionActivityDetails</Role>
      <ShortName>Stockholders' Equity - Schedule of Stock Option Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>9955556 - Disclosure - Stockholders' Equity - Schedule of Non-vested Restricted Stock Units Roll Forward (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/StockholdersEquityScheduleofNonvestedRestrictedStockUnitsRollForwardDetails</Role>
      <ShortName>Stockholders' Equity - Schedule of Non-vested Restricted Stock Units Roll Forward (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>9955557 - Disclosure - Stockholders' Equity - Share-Based Payment Arrangement, Performance Shares, Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/StockholdersEquityShareBasedPaymentArrangementPerformanceSharesActivityDetails</Role>
      <ShortName>Stockholders' Equity - Share-Based Payment Arrangement, Performance Shares, Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>9955558 - Disclosure - Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/IncomeTaxesDetails</Role>
      <ShortName>Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/IncomeTaxesTables</ParentRole>
      <Position>72</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>9955559 - Disclosure - Regulatory Capital Matters - Schedule of Actual and required capital amounts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/RegulatoryCapitalMattersScheduleofActualandrequiredcapitalamountsDetails</Role>
      <ShortName>Regulatory Capital Matters - Schedule of Actual and required capital amounts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>9955560 - Disclosure - Fair Value Measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/FairValueMeasurementsScheduleofFairValueAssetsandLiabilitiesMeasuredonRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>9955561 - Disclosure - Fair Value Measurements - Schedule of Reconciliation for Level 3 Assets Measured at Fair Value on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/FairValueMeasurementsScheduleofReconciliationforLevel3AssetsMeasuredatFairValueonaRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Reconciliation for Level 3 Assets Measured at Fair Value on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>9955562 - Disclosure - Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on a Non-recurring (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/FairValueMeasurementsScheduleofAssetsandLiabilitiesMeasuredatFairValueonaNonrecurringDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on a Non-recurring (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>9955563 - Disclosure - Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments Not Carried at Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/FairValueMeasurementsScheduleofEstimatedFairValuesofFinancialInstrumentsNotCarriedatFairValueDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments Not Carried at Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>9955564 - Disclosure - Segment Information - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/SegmentInformationNarrativeDetails</Role>
      <ShortName>Segment Information - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>9955565 - Disclosure - Segment Information - Schedule of Segment Reporting Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/SegmentInformationScheduleofSegmentReportingInformationDetails</Role>
      <ShortName>Segment Information - Schedule of Segment Reporting Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>9955566 - Disclosure - Commitments and Contingencies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/CommitmentsandContingenciesDetails</Role>
      <ShortName>Commitments and Contingencies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sunflowerbank.com/role/CommitmentsandContingencies</ParentRole>
      <Position>80</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>9955567 - Disclosure - Lease Commitments - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LeaseCommitmentsNarrativeDetails</Role>
      <ShortName>Lease Commitments - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>9955568 - Disclosure - Lease Commitments - Schedule of Leases Existing During Transition from ASC 840 to ASC 842 (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LeaseCommitmentsScheduleofLeasesExistingDuringTransitionfromASC840toASC842Details</Role>
      <ShortName>Lease Commitments - Schedule of Leases Existing During Transition from ASC 840 to ASC 842 (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>9955569 - Disclosure - Lease Commitments - Schedule of Future Undiscounted Lease Payments Due Under Non-Cancelable Operating Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LeaseCommitmentsScheduleofFutureUndiscountedLeasePaymentsDueUnderNonCancelableOperatingLeasesDetails</Role>
      <ShortName>Lease Commitments - Schedule of Future Undiscounted Lease Payments Due Under Non-Cancelable Operating Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="fcb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>9955570 - Disclosure - Lease Commitments - Schedule of Components of Total Lease Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sunflowerbank.com/role/LeaseCommitmentsScheduleofComponentsofTotalLeaseExpenseDetails</Role>
      <ShortName>Lease Commitments - Schedule of Components of Total Lease Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</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" isDefinitelyFs="true" isUsgaap="true" original="fcb-20260331.htm">fcb-20260331.htm</File>
    <File>fcb-20260331.xsd</File>
    <File>fcb-20260331_cal.xml</File>
    <File>fcb-20260331_def.xml</File>
    <File>fcb-20260331_lab.xml</File>
    <File>fcb-20260331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="2790">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="32">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
