<?xml version="1.0" encoding="utf-8"?>
<FilingSummary xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Version>2.4.0.6</Version>
  <ProcessingTime />
  <ReportFormat>Html</ReportFormat>
  <ContextCount>88</ContextCount>
  <ElementCount>132</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>50</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>9</UnitCount>
  <MyReports>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>101 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/DocumentDocumentandEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>103 - Statement - Condensed Statements of Financial Condition</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/StatementOfFinancialPositionUnclassified-SecuritiesBasedOperations</Role>
      <ShortName>Condensed Statements of Financial Condition</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>104 - Statement - Condensed Schedule of Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/StatementCondensedScheduleOfInvestments</Role>
      <ShortName>Condensed Schedule of Investments</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>105 - Statement - Condensed Schedule of Investments (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/StatementCondensedScheduleOfInvestmentsParenthetical</Role>
      <ShortName>Condensed Schedule of Investments (Parenthetical)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>106 - Statement - Condensed Statements of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/StatementOfIncomeSecuritiesBasedIncome</Role>
      <ShortName>Condensed Statements of Operations</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>107 - Statement - Condensed Statement of Changes in Partners' Capital</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/StatementOfPartnersCapital</Role>
      <ShortName>Condensed Statement of Changes in Partners' Capital</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>108 - Statement - Condensed Statement of Changes in Partners' Capital (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/StatementOfPartnersCapitalParenthetical</Role>
      <ShortName>Condensed Statement of Changes in Partners' Capital (Parenthetical)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>109 - Statement - Condensed Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/StatementOfCashFlowsIndirectSecuritiesBasedOperations</Role>
      <ShortName>Condensed Statements of Cash Flows</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>110 - Disclosure - Condensed Schedule of Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/NotesToFinancialStatementsSummaryOfInvestmentHoldingsScheduleOfInvestmentsTextBlock</Role>
      <ShortName>Condensed Schedule of Investments</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>111 - Disclosure - Organization and Business</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/NotesToFinancialStatementsOrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureTextBlock</Role>
      <ShortName>Organization and Business</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>112 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/NotesToFinancialStatementsSignificantAccountingPoliciesTextBlock</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>113 - Disclosure - Fees Paid by the Fund and Related Party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/NotesToFinancialStatementsFeesPaidAndRelatedPartyTransactionsDisclosureTextBlock</Role>
      <ShortName>Fees Paid by the Fund and Related Party Transactions</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>114 - Disclosure - Contracts and Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/NotesToFinancialStatementsLongTermContractsOrProgramsDisclosureTextBlock</Role>
      <ShortName>Contracts and Agreements</ShortName>
    </Report>
    <Report>
      <IsDefault>true</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>115 - Disclosure - Financial Instruments, Off-Balance Sheet Risks and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/NotesToFinancialStatementsDisclosureOfFairValueOffBalanceSheetRisksTextBlock</Role>
      <ShortName>Financial Instruments, Off-Balance Sheet Risks and Contingencies</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>116 - Disclosure - Financial Highlights</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/NotesToFinancialStatementsSupplementalFinancialInformationDisclosureTextBlock</Role>
      <ShortName>Financial Highlights</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>117 - Disclosure - Fair Value of Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/NotesToFinancialStatementsFairValueDisclosuresTextBlock</Role>
      <ShortName>Fair Value of Financial Instruments</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>118 - Disclosure - Recent Accounting Pronouncements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/NotesToFinancialStatementsDescriptionOfNewAccountingPronouncementsTextBlock</Role>
      <ShortName>Recent Accounting Pronouncements</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>119 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/NotesToFinancialStatementsSubsequentEventsTextBlock</Role>
      <ShortName>Subsequent Events</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>120 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/NotesToFinancialStatementsSignificantAccountingPoliciesTextBlockPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>121 - Disclosure - Financial Highlights (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/NotesToFinancialStatementsSupplementalFinancialInformationDisclosureTextBlockTables</Role>
      <ShortName>Financial Highlights (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>122 - Disclosure - Fair Value of Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/NotesToFinancialStatementsFairValueDisclosuresTextBlockTables</Role>
      <ShortName>Fair Value of Financial Instruments (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>123 - Disclosure - Organization and Business - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/DisclosureOrganizationAndBusinessAdditionalInformation</Role>
      <ShortName>Organization and Business - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>124 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformation</Role>
      <ShortName>Summary of Significant Accounting Policies - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>125 - Disclosure - Fees Paid by Fund and Related Party Transactions - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/DisclosureFeesPaidByFundAndRelatedPartyTransactionsAdditionalInformation</Role>
      <ShortName>Fees Paid by Fund and Related Party Transactions - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>126 - Disclosure - Contracts and Agreements - Additional information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/DisclosureContractsAndAgreementsAdditionalInformation</Role>
      <ShortName>Contracts and Agreements - Additional information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>127 - Disclosure - Financial Instruments, Off-Balance Sheet Risks and Contingencies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/DisclosureFinancialInstrumentsOffBalanceSheetRisksAndContingenciesAdditionalInformation</Role>
      <ShortName>Financial Instruments, Off-Balance Sheet Risks and Contingencies - Additional Information (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>128 - Disclosure - Per Unit Performance Data and Other Supplemental Financial Data (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/DisclosurePerUnitPerformanceDataAndOtherSupplementalFinancialData</Role>
      <ShortName>Per Unit Performance Data and Other Supplemental Financial Data (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>129 - Disclosure - Valuation of Securities Using Fair Value Hierarchy (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/DisclosureValuationOfSecuritiesUsingFairValueHierarchy</Role>
      <ShortName>Valuation of Securities Using Fair Value Hierarchy (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>130 - Disclosure - Fair Value of Derivative Instruments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/DisclosureFairValueOfDerivativeInstruments</Role>
      <ShortName>Fair Value of Derivative Instruments (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>131 - Disclosure - Effect of Derivative Instruments on Condensed Statements of Operations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.unitedstates12monthoilfund.com/taxonomy/role/DisclosureEffectOfDerivativeInstrumentsOnCondensedStatementsOfOperations</Role>
      <ShortName>Effect of Derivative Instruments on Condensed Statements of Operations (Detail)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs>
    <Log type="Info">Element usl_BaseFeePercentage had a mix of decimals attribute values: 5 6.</Log>
    <Log type="Info">Process Flow-Through: 103 - Statement - Condensed Statements of Financial Condition</Log>
    <Log type="Info">	Process Flow-Through: Removing column 'Jun. 30, 2011'</Log>
    <Log type="Info">	Process Flow-Through: Removing column 'Dec. 31, 2010'</Log>
    <Log type="Info">	Process Flow-Through: Removing column 'Dec. 06, 2007'</Log>
    <Log type="Info">Process Flow-Through: 104 - Statement - Condensed Schedule of Investments</Log>
    <Log type="Info">	Process Flow-Through: Removing column 'Dec. 31, 2011'</Log>
    <Log type="Info">Process Flow-Through: 105 - Statement - Condensed Schedule of Investments (Parenthetical)</Log>
    <Log type="Info">Process Flow-Through: 106 - Statement - Condensed Statements of Operations</Log>
    <Log type="Info">Process Flow-Through: 108 - Statement - Condensed Statement of Changes in Partners' Capital (Parenthetical)</Log>
    <Log type="Info">Process Flow-Through: 109 - Statement - Condensed Statements of Cash Flows</Log>
  </Logs>
  <InputFiles>
    <File>usl-20120630.xml</File>
    <File>usl-20120630.xsd</File>
    <File>usl-20120630_cal.xml</File>
    <File>usl-20120630_def.xml</File>
    <File>usl-20120630_lab.xml</File>
    <File>usl-20120630_pre.xml</File>
  </InputFiles>
  <SupplementalFiles />
  <BaseTaxonomies />
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
