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Schedule of Investments (USD $)
Mar. 31, 2015
Unrealized Gain (Loss) on Open Commodity Contracts $ (15,855,783)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
Open Futures Contracts, Long | United States  
Number of Contracts 1,349us-gaap_InvestmentOwnedBalanceContracts
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Unrealized Gain (Loss) on Open Commodity Contracts (15,855,783)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
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= country_US
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% of Partners' Capital (22.06%)usl_PercentageOfPartnersCapital
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= country_US
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL May 2015 contracts, expiring April 2015  
Number of Contracts 112us-gaap_InvestmentOwnedBalanceContracts
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= usl_NymexWtiCrudeOilFuturesClMay2015ContractsExpiringApril2015Member
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Unrealized Gain (Loss) on Open Commodity Contracts (2,120,930)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
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= country_US
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% of Partners' Capital (2.95%)usl_PercentageOfPartnersCapital
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMay2015ContractsExpiringApril2015Member
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= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL June 2015 contracts, expiring May 2015  
Number of Contracts 113us-gaap_InvestmentOwnedBalanceContracts
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= usl_NymexWtiCrudeOilFuturesClJune2015ContractsExpiringMay2015Member
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Unrealized Gain (Loss) on Open Commodity Contracts (1,941,100)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
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= usl_NymexWtiCrudeOilFuturesClJune2015ContractsExpiringMay2015Member
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% of Partners' Capital (2.70%)usl_PercentageOfPartnersCapital
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJune2015ContractsExpiringMay2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL July 2015 contracts, expiring June 2015  
Number of Contracts 112us-gaap_InvestmentOwnedBalanceContracts
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= usl_NymexWtiCrudeOilFuturesClJuly2015ContractsExpiringJune2015Member
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= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts (1,960,790)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJuly2015ContractsExpiringJune2015Member
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% of Partners' Capital (2.73%)usl_PercentageOfPartnersCapital
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJuly2015ContractsExpiringJune2015Member
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= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL August 2015 contracts, expiring July 2015  
Number of Contracts 113us-gaap_InvestmentOwnedBalanceContracts
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/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClAugust2015ContractsExpiringJuly2015Member
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= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts (2,015,910)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
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= usl_NymexWtiCrudeOilFuturesClAugust2015ContractsExpiringJuly2015Member
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% of Partners' Capital (2.80%)usl_PercentageOfPartnersCapital
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClAugust2015ContractsExpiringJuly2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL September 2015 contracts, expiring August 2015  
Number of Contracts 112us-gaap_InvestmentOwnedBalanceContracts
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/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClSeptember2015ContractsExpiringAugust2015Member
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= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts (1,812,480)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClSeptember2015ContractsExpiringAugust2015Member
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= usl_OpenFuturesContractsLongMember
% of Partners' Capital (2.52%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClSeptember2015ContractsExpiringAugust2015Member
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= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL October 2015 contracts, expiring September 2015  
Number of Contracts 112us-gaap_InvestmentOwnedBalanceContracts
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= usl_NymexWtiCrudeOilFuturesClOctober2015ContractsExpiringSeptember2015Member
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= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts (1,683,410)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
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= usl_NymexWtiCrudeOilFuturesClOctober2015ContractsExpiringSeptember2015Member
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% of Partners' Capital (2.34%)usl_PercentageOfPartnersCapital
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClOctober2015ContractsExpiringSeptember2015Member
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= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL November 2015 contracts, expiring October 2015  
Number of Contracts 112us-gaap_InvestmentOwnedBalanceContracts
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/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClNovember2015ContractsExpiringOctober2015Member
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= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts (1,458,810)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClNovember2015ContractsExpiringOctober2015Member
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= usl_OpenFuturesContractsLongMember
% of Partners' Capital (2.03%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClNovember2015ContractsExpiringOctober2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL December 2015 contracts, expiring November 2015  
Number of Contracts 113us-gaap_InvestmentOwnedBalanceContracts
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/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClDecember2015ContractsExpiringNovember2015Member
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= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts (1,161,110)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClDecember2015ContractsExpiringNovember2015Member
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% of Partners' Capital (1.61%)usl_PercentageOfPartnersCapital
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClDecember2015ContractsExpiringNovember2015Member
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= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL January 2016 contracts, expiring December 2015  
Number of Contracts 112us-gaap_InvestmentOwnedBalanceContracts
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/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJanuary2016ContractsExpiringDecember2015Member
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= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts (666,623)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJanuary2016ContractsExpiringDecember2015Member
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% of Partners' Capital (0.93%)usl_PercentageOfPartnersCapital
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJanuary2016ContractsExpiringDecember2015Member
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= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL February 2016 contracts, expiring January 2016 [Member]  
Number of Contracts 113us-gaap_InvestmentOwnedBalanceContracts
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/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClFebruary2016ContractsExpiringJanuary2016Member
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Unrealized Gain (Loss) on Open Commodity Contracts (111,900)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClFebruary2016ContractsExpiringJanuary2016Member
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% of Partners' Capital (0.16%)usl_PercentageOfPartnersCapital
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClFebruary2016ContractsExpiringJanuary2016Member
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= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL March 2016 contracts, expiring February 2016 [Member]  
Number of Contracts 113us-gaap_InvestmentOwnedBalanceContracts
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/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMarch2016ContractsExpiringFebruary2016Member
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Unrealized Gain (Loss) on Open Commodity Contracts (565,930)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMarch2016ContractsExpiringFebruary2016Member
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= usl_OpenFuturesContractsLongMember
% of Partners' Capital (0.79%)usl_PercentageOfPartnersCapital
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMarch2016ContractsExpiringFebruary2016Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL April 2016 contracts, expiring March 2016 [Member]  
Number of Contracts 112us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClApril2016ContractsExpiringMarch2016Member
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= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts (356,790)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
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= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClApril2016ContractsExpiringMarch2016Member
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% of Partners' Capital (0.50%)usl_PercentageOfPartnersCapital
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= usl_NymexWtiCrudeOilFuturesClApril2016ContractsExpiringMarch2016Member
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= usl_OpenFuturesContractsLongMember
Cash Equivalents | United States  
Market Value 50,992,391us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
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% of Partners' Capital 70.94%usl_PercentageOfPartnersCapital
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/ invest_InvestmentGeographicRegionAxis
= country_US
Cash Equivalents | United States | US Treasury Securities  
Market Value 26,992,391us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
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= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
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% of Partners' Capital 37.55%usl_PercentageOfPartnersCapital
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/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_USTreasurySecuritiesMember
Cash Equivalents | United States | US Treasury Securities | U.S. Treasury Bills 0.05%, 7/9/2015  
Principal Amount 2,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
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Market Value 1,999,753us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
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% of Partners' Capital 2.78%usl_PercentageOfPartnersCapital
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= country_US
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= us-gaap_USTreasurySecuritiesMember
Cash Equivalents | United States | US Treasury Securities | U.S.Treasury Bills 0.08%, 7/23/2015  
Principal Amount 4,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
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Market Value 3,999,058us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
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% of Partners' Capital 5.57%usl_PercentageOfPartnersCapital
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= us-gaap_USTreasurySecuritiesMember
Cash Equivalents | United States | US Treasury Securities | U.S.Treasury Bills 0.06%, 7/30/2015  
Principal Amount 2,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
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Market Value 1,999,633us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
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% of Partners' Capital 2.78%usl_PercentageOfPartnersCapital
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Cash Equivalents | United States | US Treasury Securities | U.S.Treasury Bills 0.06%, 8/6/2015  
Principal Amount 2,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
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Market Value 1,999,612us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
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% of Partners' Capital 2.78%usl_PercentageOfPartnersCapital
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= us-gaap_USTreasurySecuritiesMember
Cash Equivalents | United States | US Treasury Securities | U.S.Treasury Bills 0.07%, 8/13/2015  
Principal Amount 2,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
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Market Value 1,999,516us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
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% of Partners' Capital 2.78%usl_PercentageOfPartnersCapital
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Cash Equivalents | United States | US Treasury Securities | U.S.Treasury Bills 0.07%, 8/20/2015  
Principal Amount 1,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
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Market Value 999,746us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
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% of Partners' Capital 1.39%usl_PercentageOfPartnersCapital
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Cash Equivalents | United States | US Treasury Securities | U.S.Treasury Bills 0.07%, 8/27/2015  
Principal Amount 1,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
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Market Value 999,733us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
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% of Partners' Capital 1.39%usl_PercentageOfPartnersCapital
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Cash Equivalents | United States | US Treasury Securities | U.S.Treasury Bills 0.08%, 9/3/2015  
Principal Amount 4,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
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Market Value 3,998,708us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
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% of Partners' Capital 5.56%usl_PercentageOfPartnersCapital
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Cash Equivalents | United States | US Treasury Securities | U.S.Treasury Bills 0.10%, 9/10/2015  
Principal Amount 2,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
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