<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.19.3.a.u2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>1062</ContextCount>
  <ElementCount>813</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>241</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>8</UnitCount>
  <MyReports>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/DocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001000 - Statement - Consolidated Statements of Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/ConsolidatedStatementsOfIncome</Role>
      <ShortName>Consolidated Statements of Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1002000 - Statement - Consolidated Statements of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/ConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>Consolidated Statements of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1003000 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1003501 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1004000 - Statement - Consolidated Statements of Changes in Stockholder's Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/ConsolidatedStatementsOfChangesInStockholdersEquity</Role>
      <ShortName>Consolidated Statements of Changes in Stockholder's Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1005000 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/ConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Summary of Significant Accounting Policies and Nature of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SummaryOfSignificantAccountingPoliciesAndNatureOfOperations</Role>
      <ShortName>Summary of Significant Accounting Policies and Nature of Operations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2102100 - Disclosure - Securities1</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/Securities1</Role>
      <ShortName>Securities1</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2103100 - Disclosure - Loans and Allowance for Loan Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLosses</Role>
      <ShortName>Loans and Allowance for Loan Losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2105100 - Disclosure - Goodwill and Other Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/GoodwillAndOtherIntangibleAssets</Role>
      <ShortName>Goodwill and Other Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2105100 - Disclosure - Other Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/OtherAssets</Role>
      <ShortName>Other Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2106100 - Disclosure - Variable Interest Entities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/VariableInterestEntities</Role>
      <ShortName>Variable Interest Entities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2108100 - Disclosure - Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/Deposits</Role>
      <ShortName>Deposits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2108100 - Disclosure - Securities Financing Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SecuritiesFinancingAgreements</Role>
      <ShortName>Securities Financing Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2110100 - Disclosure - Commercial Paper and Other Short-Term Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/CommercialPaperAndOtherShortTermBorrowings</Role>
      <ShortName>Commercial Paper and Other Short-Term Borrowings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2111100 - Disclosure - Long-Term Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LongTermDebt</Role>
      <ShortName>Long-Term Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2114100 - Disclosure - Fair Value Measurement and Fair Value of Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/FairValueMeasurementAndFairValueOfFinancialInstruments</Role>
      <ShortName>Fair Value Measurement and Fair Value of Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2115100 - Disclosure - Derivative Instruments and Other Financial Instruments Used For Hedging</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/DerivativeInstrumentsAndOtherFinancialInstrumentsUsedForHedging</Role>
      <ShortName>Derivative Instruments and Other Financial Instruments Used For Hedging</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2118100 - Disclosure - Accumulated Other Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/AccumulatedOtherComprehensiveIncome</Role>
      <ShortName>Accumulated Other Comprehensive Income</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2119100 - Disclosure - Management Stock Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/ManagementStockPlans</Role>
      <ShortName>Management Stock Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2120100 - Disclosure - Employee Pension and Other Postretirement Benefits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefits</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2121100 - Disclosure - Other Noninterest Income and Noninterest Expense</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/OtherNoninterestIncomeAndNoninterestExpense</Role>
      <ShortName>Other Noninterest Income and Noninterest Expense</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2122100 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2123100 - Disclosure - Regulatory Capital Requirements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/RegulatoryCapitalRequirements</Role>
      <ShortName>Regulatory Capital Requirements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2124100 - Disclosure - Restrictions on Cash and Due from Banks, Securities, Loans and Dividends</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/RestrictionsOnCashAndDueFromBanksSecuritiesLoansAndDividends</Role>
      <ShortName>Restrictions on Cash and Due from Banks, Securities, Loans and Dividends</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2125100 - Disclosure - Commitments, Contingencies and Guarantees</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/CommitmentsContingenciesAndGuarantees</Role>
      <ShortName>Commitments, Contingencies and Guarantees</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2126100 - Disclosure - Related Party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/RelatedPartyTransactions</Role>
      <ShortName>Related Party Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2127100 - Disclosure - Business Segments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/BusinessSegments</Role>
      <ShortName>Business Segments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2128100 - Disclosure - Condensed MUFG Americas Holdings Corporation Unconsolidated Financial Statements (Parent Company)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/CondensedMufgAmericasHoldingsCorporationUnconsolidatedFinancialStatementsParentCompany</Role>
      <ShortName>Condensed MUFG Americas Holdings Corporation Unconsolidated Financial Statements (Parent Company)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2201201 - Disclosure - Summary of Significant Accounting Policies and Nature of Operations (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SummaryOfSignificantAccountingPoliciesAndNatureOfOperationsPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies and Nature of Operations (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/SummaryOfSignificantAccountingPoliciesAndNatureOfOperations</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2302301 - Disclosure - Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SecuritiesTables</Role>
      <ShortName>Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/Securities1</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2303301 - Disclosure - Loans and Allowance for Loan Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesTables</Role>
      <ShortName>Loans and Allowance for Loan Losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/LoansAndAllowanceForLoanLosses</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2305301 - Disclosure - Goodwill and Other Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/GoodwillAndOtherIntangibleAssetsTables</Role>
      <ShortName>Goodwill and Other Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/GoodwillAndOtherIntangibleAssets</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2305301 - Disclosure - Other Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/OtherAssetsTables</Role>
      <ShortName>Other Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/OtherAssets</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2306301 - Disclosure - Variable Interest Entities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/VariableInterestEntitiesTables</Role>
      <ShortName>Variable Interest Entities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/VariableInterestEntities</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2308301 - Disclosure - Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/DepositsTables</Role>
      <ShortName>Deposits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/Deposits</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2308301 - Disclosure - Securities Financing Agreements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SecuritiesFinancingAgreementsTables</Role>
      <ShortName>Securities Financing Agreements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/SecuritiesFinancingAgreements</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2310301 - Disclosure - Commercial Paper and Other Short-Term Borrowings (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/CommercialPaperAndOtherShortTermBorrowingsTables</Role>
      <ShortName>Commercial Paper and Other Short-Term Borrowings (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/CommercialPaperAndOtherShortTermBorrowings</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2311301 - Disclosure - Long-Term Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LongTermDebtTables</Role>
      <ShortName>Long-Term Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/LongTermDebt</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2314301 - Disclosure - Fair Value Measurement and Fair Value of Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/FairValueMeasurementAndFairValueOfFinancialInstrumentsTables</Role>
      <ShortName>Fair Value Measurement and Fair Value of Financial Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/FairValueMeasurementAndFairValueOfFinancialInstruments</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2315301 - Disclosure - Derivative Instruments and Other Financial Instruments Used For Hedging (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/DerivativeInstrumentsAndOtherFinancialInstrumentsUsedForHedgingTables</Role>
      <ShortName>Derivative Instruments and Other Financial Instruments Used For Hedging (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/DerivativeInstrumentsAndOtherFinancialInstrumentsUsedForHedging</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2318301 - Disclosure - Accumulated Other Comprehensive Income (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/AccumulatedOtherComprehensiveIncomeTables</Role>
      <ShortName>Accumulated Other Comprehensive Income (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/AccumulatedOtherComprehensiveIncome</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2319301 - Disclosure - Management Stock Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/ManagementStockPlansTables</Role>
      <ShortName>Management Stock Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/ManagementStockPlans</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2320301 - Disclosure - Employee Pension and Other Postretirement Benefits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefitsTables</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefits</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2321301 - Disclosure - Other Noninterest Income and Noninterest Expense (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/OtherNoninterestIncomeAndNoninterestExpenseTables</Role>
      <ShortName>Other Noninterest Income and Noninterest Expense (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/OtherNoninterestIncomeAndNoninterestExpense</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2322301 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/IncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/IncomeTaxes</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2323301 - Disclosure - Regulatory Capital Requirements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/RegulatoryCapitalRequirementsTables</Role>
      <ShortName>Regulatory Capital Requirements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/RegulatoryCapitalRequirements</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2325301 - Disclosure - Commitments, Contingencies and Guarantees (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/CommitmentsContingenciesAndGuaranteesTables</Role>
      <ShortName>Commitments, Contingencies and Guarantees (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/CommitmentsContingenciesAndGuarantees</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2327301 - Disclosure - Business Segments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/BusinessSegmentsTables</Role>
      <ShortName>Business Segments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/BusinessSegments</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2328301 - Disclosure - Condensed MUFG Americas Holdings Corporation Unconsolidated Financial Statements (Parent Company) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/CondensedMufgAmericasHoldingsCorporationUnconsolidatedFinancialStatementsParentCompanyTables</Role>
      <ShortName>Condensed MUFG Americas Holdings Corporation Unconsolidated Financial Statements (Parent Company) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/CondensedMufgAmericasHoldingsCorporationUnconsolidatedFinancialStatementsParentCompany</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2401403 - Disclosure - Summary of Significant Accounting Policies and Nature of Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SummaryOfSignificantAccountingPoliciesAndNatureOfOperationsDetails</Role>
      <ShortName>Summary of Significant Accounting Policies and Nature of Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/SummaryOfSignificantAccountingPoliciesAndNatureOfOperationsPolicies</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2401404 - Disclosure - - Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/NetPeriodicPensionCostAndNetPeriodicPostretirementBenefitCostDetails</Role>
      <ShortName>- Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2402402 - Disclosure - Correction of Prior Period Amounts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/CorrectionOfPriorPeriodAmountsDetails</Role>
      <ShortName>Correction of Prior Period Amounts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2402402 - Disclosure - Securities - Schedule of Available For Sale Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SecuritiesScheduleOfAvailableForSaleSecuritiesDetails</Role>
      <ShortName>Securities - Schedule of Available For Sale Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2402403 - Disclosure - Securities - Schedule of Continuous Unrealized Loss Position (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SecuritiesScheduleOfContinuousUnrealizedLossPositionDetails</Role>
      <ShortName>Securities - Schedule of Continuous Unrealized Loss Position (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2402404 - Disclosure - Securities - Fair Value of Debt Securities AFS Contractual Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SecuritiesFairValueOfDebtSecuritiesAfsContractualMaturityDetails</Role>
      <ShortName>Securities - Fair Value of Debt Securities AFS Contractual Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2402405 - Disclosure - Securities  - Schedule of Held to Maturity Securities Recognized and Not Recognized in Other Comprehensive Income (OCI) and Fair Values (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SecuritiesScheduleOfHeldToMaturitySecuritiesRecognizedAndNotRecognizedInOtherComprehensiveIncomeOciAndFairValuesDetails</Role>
      <ShortName>Securities  - Schedule of Held to Maturity Securities Recognized and Not Recognized in Other Comprehensive Income (OCI) and Fair Values (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2402406 - Disclosure - Securities - Unrealized Gains/Loss in OCI (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SecuritiesUnrealizedGainsLossInOciDetails</Role>
      <ShortName>Securities - Unrealized Gains/Loss in OCI (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2402407 - Disclosure - Securities - Held to Maturity Continuous Unrealized Loss Position (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SecuritiesHeldToMaturityContinuousUnrealizedLossPositionDetails</Role>
      <ShortName>Securities - Held to Maturity Continuous Unrealized Loss Position (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2402408 - Disclosure - Securities - Carrying Amount and Fair Value Securities Held to Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SecuritiesCarryingAmountAndFairValueSecuritiesHeldToMaturityDetails</Role>
      <ShortName>Securities - Carrying Amount and Fair Value Securities Held to Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2402409 - Disclosure - Securities - Securities Pledged as Collateral (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SecuritiesSecuritiesPledgedAsCollateralDetails</Role>
      <ShortName>Securities - Securities Pledged as Collateral (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2403402 - Disclosure - Loans and Allowance for Loan Losses - Outstanding Balance of Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesOutstandingBalanceOfLoansDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Outstanding Balance of Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2403403 - Disclosure - Loans and Allowance for Loan Losses - Reconciliation of Changes in Allowance for Loan Losses by Portfolio Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesReconciliationOfChangesInAllowanceForLoanLossesByPortfolioSegmentDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Reconciliation of Changes in Allowance for Loan Losses by Portfolio Segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2403404 - Disclosure - Loans and Allowance for Loan Losses - Loan Losses and Related Loan Balances by Portfolio Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesLoanLossesAndRelatedLoanBalancesByPortfolioSegmentDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Loan Losses and Related Loan Balances by Portfolio Segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2403405 - Disclosure - Loans and Allowance for Loan Losses - Nonaccural Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesNonaccuralLoansDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Nonaccural Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2403406 - Disclosure - Loans and Allowance for Loan Losses - Balance of Loans Held for Investment, by Class (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesBalanceOfLoansHeldForInvestmentByClassDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Balance of Loans Held for Investment, by Class (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2403407 - Disclosure - Loans and Allowance for Loan Losses - Purchased Credit-Impaired Loans Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesPurchasedCreditImpairedLoansSegmentDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Purchased Credit-Impaired Loans Segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2403408 - Disclosure - Loans and Allowance for Loan Losses - Loans in Consumer Portfolio Segment and Purchased Credit Impaired Loans Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesLoansInConsumerPortfolioSegmentAndPurchasedCreditImpairedLoansSegmentDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Loans in Consumer Portfolio Segment and Purchased Credit Impaired Loans Segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2403409 - Disclosure - Loans and Allowance for Loan Losses  - Loans in Consumer Portfolio Segment Based on Refreshed FICO Scores (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesLoansInConsumerPortfolioSegmentBasedOnRefreshedFicoScoresDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses  - Loans in Consumer Portfolio Segment Based on Refreshed FICO Scores (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2403410 - Disclosure - Loans and Allowance for Loan Losses  - Loans in Consumer Portfolio Segment Based on Refreshed LTV ratios (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesLoansInConsumerPortfolioSegmentBasedOnRefreshedLtvRatiosDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses  - Loans in Consumer Portfolio Segment Based on Refreshed LTV ratios (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2403411 - Disclosure - Loans and Allowance for Loan Losses - Summary of Recorded Investment in TDR's (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesSummaryOfRecordedInvestmentInTdrsDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Summary of Recorded Investment in TDR's (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2403412 - Disclosure - Loans and Allowance for Loan Losses - Pre and Post Modification Outstanding Recorded Investment Amounts of TDRs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesPreAndPostModificationOutstandingRecordedInvestmentAmountsOfTdrsDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Pre and Post Modification Outstanding Recorded Investment Amounts of TDRs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2403413 - Disclosure - Loans and Allowance for Loan Losses - Recorded Investment Amounts of TDRs at Date of Default (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesRecordedInvestmentAmountsOfTdrsAtDateOfDefaultDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Recorded Investment Amounts of TDRs at Date of Default (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2403414 - Disclosure - Loans and Allowance for Loan Losses - Impaired Loans by Class (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesImpairedLoansByClassDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Impaired Loans by Class (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2403415 - Disclosure - Loans and Allowance for Loan Losses - Average Recorded Investment in Impaired Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesAverageRecordedInvestmentInImpairedLoansDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Average Recorded Investment in Impaired Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2403417 - Disclosure - Loans and Allowance for Loan Losses - Accretable Yield for Purchased Credit-Impaired Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LoansAndAllowanceForLoanLossesAccretableYieldForPurchasedCreditImpairedLoansDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Accretable Yield for Purchased Credit-Impaired Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - Goodwill and Other Intangible Assets - Schedule of Goodwill (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/GoodwillAndOtherIntangibleAssetsScheduleOfGoodwillDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets - Schedule of Goodwill (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - Other Assets - Premises and Equipment Carried at Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/OtherAssetsPremisesAndEquipmentCarriedAtCostDetails</Role>
      <ShortName>Other Assets - Premises and Equipment Carried at Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>2405403 - Disclosure - Goodwill and Other Intangible Assets - Identifiable Intangible Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/GoodwillAndOtherIntangibleAssetsIdentifiableIntangibleAssetsDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets - Identifiable Intangible Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>2405404 - Disclosure - Goodwill and Other Intangible Assets - Estimated Future Amortization Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/GoodwillAndOtherIntangibleAssetsEstimatedFutureAmortizationExpenseDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets - Estimated Future Amortization Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>2405406 - Disclosure - Other Assets - Other Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/OtherAssetsOtherAssetsDetails</Role>
      <ShortName>Other Assets - Other Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>2406402 - Disclosure - Variable Interest Entities - Assets and Liabilities of Consolidated VIEs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/VariableInterestEntitiesAssetsAndLiabilitiesOfConsolidatedViesDetails</Role>
      <ShortName>Variable Interest Entities - Assets and Liabilities of Consolidated VIEs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>2406403 - Disclosure - Variable Interest Entities - Carrying Amounts related to Unconsolidated VIEs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/VariableInterestEntitiesCarryingAmountsRelatedToUnconsolidatedViesDetails</Role>
      <ShortName>Variable Interest Entities - Carrying Amounts related to Unconsolidated VIEs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>2406404 - Disclosure - Variable Interest Entities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/VariableInterestEntitiesDetails</Role>
      <ShortName>Variable Interest Entities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/VariableInterestEntitiesTables</ParentRole>
      <Position>85</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>2408402 - Disclosure - Deposits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/DepositsDetails</Role>
      <ShortName>Deposits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/DepositsTables</ParentRole>
      <Position>86</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>2408402 - Disclosure - Securities Financing Agreements (Gross Obligations to Repurchase, Securities Loaned, and Class of Collateral Pledged) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SecuritiesFinancingAgreementsGrossObligationsToRepurchaseSecuritiesLoanedAndClassOfCollateralPledgedDetails</Role>
      <ShortName>Securities Financing Agreements (Gross Obligations to Repurchase, Securities Loaned, and Class of Collateral Pledged) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/SecuritiesFinancingAgreementsTables</ParentRole>
      <Position>87</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>2408403 - Disclosure - Deposits - Schedule of Maturities of Time Deposits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/DepositsScheduleOfMaturitiesOfTimeDepositsDetails</Role>
      <ShortName>Deposits - Schedule of Maturities of Time Deposits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>2408403 - Disclosure - Securities Financing Agreements (Offsetting Financing Assets and Liabilities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/SecuritiesFinancingAgreementsOffsettingFinancingAssetsAndLiabilitiesDetails</Role>
      <ShortName>Securities Financing Agreements (Offsetting Financing Assets and Liabilities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/SecuritiesFinancingAgreementsTables</ParentRole>
      <Position>89</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>2410402 - Disclosure - Commercial Paper and Other Short-Term Borrowings - Summary of Commercial Paper and Other Short-Term Borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/CommercialPaperAndOtherShortTermBorrowingsSummaryOfCommercialPaperAndOtherShortTermBorrowingsDetails</Role>
      <ShortName>Commercial Paper and Other Short-Term Borrowings - Summary of Commercial Paper and Other Short-Term Borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>2410403 - Disclosure - Commercial Paper and Other Short-Term Borrowings - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/CommercialPaperAndOtherShortTermBorrowingsNarrativeDetails</Role>
      <ShortName>Commercial Paper and Other Short-Term Borrowings - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/CommercialPaperAndOtherShortTermBorrowingsTables</ParentRole>
      <Position>91</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>2411402 - Disclosure - Long-Term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/LongTermDebtDetails</Role>
      <ShortName>Long-Term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/LongTermDebtTables</ParentRole>
      <Position>92</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>2414402 - Disclosure - Fair Value Measurement and Fair Value of Financial Instruments - Fair Value Measurements on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/FairValueMeasurementAndFairValueOfFinancialInstrumentsFairValueMeasurementsOnRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurement and Fair Value of Financial Instruments - Fair Value Measurements on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>2414403 - Disclosure - Fair Value Measurement and Fair Value of Financial Instruments - Reconciliation of Assets and Liabilities on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/FairValueMeasurementAndFairValueOfFinancialInstrumentsReconciliationOfAssetsAndLiabilitiesOnRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurement and Fair Value of Financial Instruments - Reconciliation of Assets and Liabilities on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>2414404 - Disclosure - Fair Value Measurement and Fair Value of Financial Instruments - Level 3 Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/FairValueMeasurementAndFairValueOfFinancialInstrumentsLevel3AssetsAndLiabilitiesDetails</Role>
      <ShortName>Fair Value Measurement and Fair Value of Financial Instruments - Level 3 Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>2414405 - Disclosure - Fair Value Measurement and Fair Value of Financial Instruments - Assets Measured on Nonrecurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/FairValueMeasurementAndFairValueOfFinancialInstrumentsAssetsMeasuredOnNonrecurringBasisDetails</Role>
      <ShortName>Fair Value Measurement and Fair Value of Financial Instruments - Assets Measured on Nonrecurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>2414406 - Disclosure - Fair Value Measurement and Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/FairValueMeasurementAndFairValueOfFinancialInstrumentsCarryingAmountAndEstimatedFairValueOfFinancialInstrumentsDetails</Role>
      <ShortName>Fair Value Measurement and Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>2415402 - Disclosure - Derivative Instruments and Other Financial Instruments Used For Hedging - Notional and Fair Value Amounts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/DerivativeInstrumentsAndOtherFinancialInstrumentsUsedForHedgingNotionalAndFairValueAmountsDetails</Role>
      <ShortName>Derivative Instruments and Other Financial Instruments Used For Hedging - Notional and Fair Value Amounts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>2415403 - Disclosure - Derivative Instruments and Other Financial Instruments Used For Hedging - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/DerivativeInstrumentsAndOtherFinancialInstrumentsUsedForHedgingAdditionalInformationDetails</Role>
      <ShortName>Derivative Instruments and Other Financial Instruments Used For Hedging - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>2415404 - Disclosure - Derivative Instruments and Other Financial Instruments Used For Hedging - Derivatives Designated as Cash Flow Hedges (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/DerivativeInstrumentsAndOtherFinancialInstrumentsUsedForHedgingDerivativesDesignatedAsCashFlowHedgesDetails</Role>
      <ShortName>Derivative Instruments and Other Financial Instruments Used For Hedging - Derivatives Designated as Cash Flow Hedges (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>100</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>2415405 - Disclosure - Derivative Instruments and Other Financial Instruments Used For Hedging - Amount of Net Gains (Losses) for Derivatives Classified as Trading (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/DerivativeInstrumentsAndOtherFinancialInstrumentsUsedForHedgingAmountOfNetGainsLossesForDerivativesClassifiedAsTradingDetails</Role>
      <ShortName>Derivative Instruments and Other Financial Instruments Used For Hedging - Amount of Net Gains (Losses) for Derivatives Classified as Trading (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>2418402 - Disclosure - Accumulated Other Comprehensive Income - AOCI and Related Tax Effect (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/AccumulatedOtherComprehensiveIncomeAociAndRelatedTaxEffectDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income - AOCI and Related Tax Effect (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>2418403 - Disclosure - Accumulated Other Comprehensive Income - Accumulated Other Comprehensive Loss Balances (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/AccumulatedOtherComprehensiveIncomeAccumulatedOtherComprehensiveLossBalancesDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income - Accumulated Other Comprehensive Loss Balances (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>103</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>2419402 - Disclosure - Management Stock Plans - Bonus Stock Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/ManagementStockPlansBonusStockPlansDetails</Role>
      <ShortName>Management Stock Plans - Bonus Stock Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>104</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>2419403 - Disclosure - Management Stock Plans - Rollforward of RSUs under Stock Bonus Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/ManagementStockPlansRollforwardOfRsusUnderStockBonusPlanDetails</Role>
      <ShortName>Management Stock Plans - Rollforward of RSUs under Stock Bonus Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>105</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>2419404 - Disclosure - Management Stock Plans - Compensation Costs, Tax Benefit and Unrecognized Compensation Costs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/ManagementStockPlansCompensationCostsTaxBenefitAndUnrecognizedCompensationCostsDetails</Role>
      <ShortName>Management Stock Plans - Compensation Costs, Tax Benefit and Unrecognized Compensation Costs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>106</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>2420402 - Disclosure - Employee Pension and Other Postretirement Benefits - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefitsAdditionalInformationDetails</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>107</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R108.htm</HtmlFileName>
      <LongName>2420403 - Disclosure - Employee Pension and Other Postretirement Benefits - Plan Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefitsPlanAssetsDetails</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits - Plan Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>108</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R109.htm</HtmlFileName>
      <LongName>2420404 - Disclosure - Employee Pension and Other Postretirement Benefits - Fair Value Hierarchy of Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefitsFairValueHierarchyOfAssetsDetails</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits - Fair Value Hierarchy of Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>109</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R110.htm</HtmlFileName>
      <LongName>2420405 - Disclosure - Employee Pension and Other Postretirement Benefits - Investments Measured at Net Asset Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefitsInvestmentsMeasuredAtNetAssetValueDetails</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits - Investments Measured at Net Asset Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>110</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R111.htm</HtmlFileName>
      <LongName>2420406 - Disclosure - Employee Pension and Other Postretirement Benefits - Changes in Benefit Obligation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefitsChangesInBenefitObligationDetails</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits - Changes in Benefit Obligation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>111</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R112.htm</HtmlFileName>
      <LongName>2420407 - Disclosure - Employee Pension and Other Postretirement Benefits - Changes in AOCI (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefitsChangesInAociDetails</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits - Changes in AOCI (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>112</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R113.htm</HtmlFileName>
      <LongName>2420408 - Disclosure - Employee Pension and Other Postretirement Benefits - AOCI Loss for Pension (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefitsAociLossForPensionDetails</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits - AOCI Loss for Pension (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>113</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R114.htm</HtmlFileName>
      <LongName>2420409 - Disclosure - Employee Pension and Other Postretirement Benefits - Pension and Post retirement Benefit Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefitsPensionAndPostRetirementBenefitPaymentsDetails</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits - Pension and Post retirement Benefit Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>114</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R115.htm</HtmlFileName>
      <LongName>2420410 - Disclosure - Employee Pension and Other Postretirement Benefits - Periodic Benefit Costs, Forecasted (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefitsPeriodicBenefitCostsForecastedDetails</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits - Periodic Benefit Costs, Forecasted (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>115</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R116.htm</HtmlFileName>
      <LongName>2420411 - Disclosure - Employee Pension and Other Postretirement Benefits - Weighted Average Assumptions, PV of Benefit Obligations and Net Periodic Benefit Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefitsWeightedAverageAssumptionsPvOfBenefitObligationsAndNetPeriodicBenefitCostDetails</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits - Weighted Average Assumptions, PV of Benefit Obligations and Net Periodic Benefit Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>116</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R117.htm</HtmlFileName>
      <LongName>2420412 - Disclosure - Employee Pension and Other Postretirement Benefits - Weighted Average Healthcare Cost Trend Rates (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefitsWeightedAverageHealthcareCostTrendRatesDetails</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits - Weighted Average Healthcare Cost Trend Rates (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>117</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R118.htm</HtmlFileName>
      <LongName>2420414 - Disclosure - Employee Pension and Other Postretirement Benefits - Executive Supplemental Benefit Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/EmployeePensionAndOtherPostretirementBenefitsExecutiveSupplementalBenefitPlansDetails</Role>
      <ShortName>Employee Pension and Other Postretirement Benefits - Executive Supplemental Benefit Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>118</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R119.htm</HtmlFileName>
      <LongName>2421402 - Disclosure - Other Noninterest Income and Noninterest Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/OtherNoninterestIncomeAndNoninterestExpenseDetails</Role>
      <ShortName>Other Noninterest Income and Noninterest Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/OtherNoninterestIncomeAndNoninterestExpenseTables</ParentRole>
      <Position>119</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R120.htm</HtmlFileName>
      <LongName>2422402 - Disclosure - Income Taxes - Analysis of the Effective Tax Rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/IncomeTaxesAnalysisOfEffectiveTaxRateDetails</Role>
      <ShortName>Income Taxes - Analysis of the Effective Tax Rate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>120</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R121.htm</HtmlFileName>
      <LongName>2422403 - Disclosure - Income Taxes - Components of Income Tax Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/IncomeTaxesComponentsOfIncomeTaxExpenseDetails</Role>
      <ShortName>Income Taxes - Components of Income Tax Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>121</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R122.htm</HtmlFileName>
      <LongName>2422404 - Disclosure - Income Taxes - Net Deferred Tax Balances (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/IncomeTaxesNetDeferredTaxBalancesDetails</Role>
      <ShortName>Income Taxes - Net Deferred Tax Balances (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>122</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R123.htm</HtmlFileName>
      <LongName>2422405 - Disclosure - Income Taxes - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/IncomeTaxesAdditionalInformationDetails</Role>
      <ShortName>Income Taxes - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>123</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R124.htm</HtmlFileName>
      <LongName>2422406 - Disclosure - Income Taxes - Unrecognized Tax Positions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/IncomeTaxesUnrecognizedTaxPositionsDetails</Role>
      <ShortName>Income Taxes - Unrecognized Tax Positions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>124</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R125.htm</HtmlFileName>
      <LongName>2423402 - Disclosure - Regulatory Capital Requirements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/RegulatoryCapitalRequirementsDetails</Role>
      <ShortName>Regulatory Capital Requirements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/RegulatoryCapitalRequirementsTables</ParentRole>
      <Position>125</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R126.htm</HtmlFileName>
      <LongName>2424401 - Disclosure - Restrictions on Cash and Due from Banks, Securities, Loans and Dividends - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/RestrictionsOnCashAndDueFromBanksSecuritiesLoansAndDividendsAdditionalInformationDetails</Role>
      <ShortName>Restrictions on Cash and Due from Banks, Securities, Loans and Dividends - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>126</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R127.htm</HtmlFileName>
      <LongName>2425402 - Disclosure - Commitments, Contingencies and Guarantees (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/CommitmentsContingenciesAndGuaranteesDetails</Role>
      <ShortName>Commitments, Contingencies and Guarantees (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/CommitmentsContingenciesAndGuaranteesTables</ParentRole>
      <Position>127</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R128.htm</HtmlFileName>
      <LongName>2426401 - Disclosure - Related Party Transactions - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/RelatedPartyTransactionsNarrativeDetails</Role>
      <ShortName>Related Party Transactions - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>128</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R129.htm</HtmlFileName>
      <LongName>2426402 - Disclosure - - Assets and Liabilities with Affiliates (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/AssetsAndLiabilitiesWithAffiliatesDetails</Role>
      <ShortName>- Assets and Liabilities with Affiliates (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>129</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R130.htm</HtmlFileName>
      <LongName>2426403 - Disclosure - - Revenues and Expenses with Affiliates (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/RevenuesAndExpensesWithAffiliatesDetails</Role>
      <ShortName>- Revenues and Expenses with Affiliates (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>130</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R131.htm</HtmlFileName>
      <LongName>2427402 - Disclosure - Business Segments - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/BusinessSegmentsAdditionalInformationDetails</Role>
      <ShortName>Business Segments - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>131</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R132.htm</HtmlFileName>
      <LongName>2427403 - Disclosure - Business Segments - Schedule of Segments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/BusinessSegmentsScheduleOfSegmentsDetails</Role>
      <ShortName>Business Segments - Schedule of Segments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>132</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R133.htm</HtmlFileName>
      <LongName>2428402 - Disclosure - Condensed MUFG Americas Holdings Corporation Unconsolidated Financial Statements (Parent Company) - Balance Sheet (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/CondensedMufgAmericasHoldingsCorporationUnconsolidatedFinancialStatementsParentCompanyBalanceSheetDetails</Role>
      <ShortName>Condensed MUFG Americas Holdings Corporation Unconsolidated Financial Statements (Parent Company) - Balance Sheet (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/CondensedMufgAmericasHoldingsCorporationUnconsolidatedFinancialStatementsParentCompanyTables</ParentRole>
      <Position>133</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R134.htm</HtmlFileName>
      <LongName>2428403 - Disclosure - Condensed MUFG Americas Holdings Corporation Unconsolidated Financial Statements (Parent Company) - Statements of Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/CondensedMufgAmericasHoldingsCorporationUnconsolidatedFinancialStatementsParentCompanyStatementsOfIncomeDetails</Role>
      <ShortName>Condensed MUFG Americas Holdings Corporation Unconsolidated Financial Statements (Parent Company) - Statements of Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/CondensedMufgAmericasHoldingsCorporationUnconsolidatedFinancialStatementsParentCompanyTables</ParentRole>
      <Position>134</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R135.htm</HtmlFileName>
      <LongName>2428404 - Disclosure - Condensed MUFG Americas Holdings Corporation Unconsolidated Financial Statements (Parent Company) - Statement of Cash Flows (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://mufg-americas.com/role/CondensedMufgAmericasHoldingsCorporationUnconsolidatedFinancialStatementsParentCompanyStatementOfCashFlowsDetails</Role>
      <ShortName>Condensed MUFG Americas Holdings Corporation Unconsolidated Financial Statements (Parent Company) - Statement of Cash Flows (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://mufg-americas.com/role/CondensedMufgAmericasHoldingsCorporationUnconsolidatedFinancialStatementsParentCompanyTables</ParentRole>
      <Position>135</Position>
    </Report>
    <Report instance="ub-20191231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9999.htm</HtmlFileName>
      <LongName>Uncategorized Items - ub-20191231.xml</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/role/uncategorizedFacts</Role>
      <ShortName>Uncategorized Items - ub-20191231.xml</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>136</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>ub-20191231.xml</File>
    <File>ub-20191231.xsd</File>
    <File>ub-20191231_cal.xml</File>
    <File>ub-20191231_def.xml</File>
    <File>ub-20191231_lab.xml</File>
    <File>ub-20191231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/invest/2013-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2019-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
