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Schedule of Investments (USD $)
Dec. 31, 2014
Dec. 31, 2013
Unrealized Gain (Loss) on Open Commodity Contracts $ (17,096,060)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts $ 1,642,450usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
Open Futures Contracts, Long | United States    
Number of Contracts 658us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Unrealized Gain (Loss) on Open Commodity Contracts (17,096,060)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		 [1] 1,642,450usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		 [2]
% of Partners' Capital (45.92%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		 [1] 2.64%usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		 [2]
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL February 2014 contracts, expiring January 2014    
Number of Contracts   55us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClFebruary2014ContractsExpiringJanuary2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts   184,140usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClFebruary2014ContractsExpiringJanuary2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
% of Partners' Capital   0.30%usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClFebruary2014ContractsExpiringJanuary2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL March 2014 contracts, expiring February 2014    
Number of Contracts   54us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMarch2014ContractsExpiringFebruary2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts   95,360usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMarch2014ContractsExpiringFebruary2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
% of Partners' Capital   0.15%usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMarch2014ContractsExpiringFebruary2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL April 2014 contracts, expiring March 2014    
Number of Contracts   54us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClApril2014ContractsExpiringMarch2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts   404,380usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClApril2014ContractsExpiringMarch2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
% of Partners' Capital   0.65%usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClApril2014ContractsExpiringMarch2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL May 2014 contracts, expiring April 2014    
Number of Contracts   54us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMay2014ContractsExpiringApril2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts   313,310usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMay2014ContractsExpiringApril2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
% of Partners' Capital   0.51%usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMay2014ContractsExpiringApril2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL June 2014 contracts, expiring May 2014    
Number of Contracts   54us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJune2014ContractsExpiringMay2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts   269,150usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJune2014ContractsExpiringMay2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
% of Partners' Capital   0.43%usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJune2014ContractsExpiringMay2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL July 2014 contracts, expiring June 2014    
Number of Contracts   54us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJuly2014ContractsExpiringJune2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts   261,410usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJuly2014ContractsExpiringJune2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
% of Partners' Capital   0.42%usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJuly2014ContractsExpiringJune2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL August 2014 contracts, expiring July 2014    
Number of Contracts   54us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClAugust2014ContractsExpiringJuly2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts   120,620usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClAugust2014ContractsExpiringJuly2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
% of Partners' Capital   0.19%usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClAugust2014ContractsExpiringJuly2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL September 2014 contracts, expiring August 2014    
Number of Contracts   54us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClSeptember2014ContractsExpiringAugust2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts   47,050usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClSeptember2014ContractsExpiringAugust2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
% of Partners' Capital   0.08%usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClSeptember2014ContractsExpiringAugust2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL October 2014 contracts, expiring September 2014    
Number of Contracts   54us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClOctober2014ContractsExpiringSeptember2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts   (93,940)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClOctober2014ContractsExpiringSeptember2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
% of Partners' Capital   (0.15%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClOctober2014ContractsExpiringSeptember2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL November 2014 contracts, expiring October 2014    
Number of Contracts   54us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClNovember2014ContractsExpiringOctober2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts   (79,520)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClNovember2014ContractsExpiringOctober2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
% of Partners' Capital   (0.13%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClNovember2014ContractsExpiringOctober2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL December 2014 contracts, expiring November 2014    
Number of Contracts   54us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClDecember2014ContractsExpiringNovember2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts   94,480usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClDecember2014ContractsExpiringNovember2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
% of Partners' Capital   0.15%usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClDecember2014ContractsExpiringNovember2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL January 2015 contracts, expiring December 2014    
Number of Contracts   54us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJanuary2015ContractsExpiringDecember2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Unrealized Gain (Loss) on Open Commodity Contracts   26,010usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJanuary2015ContractsExpiringDecember2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
% of Partners' Capital   0.04%usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJanuary2015ContractsExpiringDecember2014Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL February 2015 contracts, expiring January 2015    
Number of Contracts 54us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClFebruary2015ContractsExpiringJanuary2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Unrealized Gain (Loss) on Open Commodity Contracts (1,577,330)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClFebruary2015ContractsExpiringJanuary2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
% of Partners' Capital (4.24%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClFebruary2015ContractsExpiringJanuary2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL March 2015 contracts, expiring February 2015    
Number of Contracts 55us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMarch2015ContractsExpiringFebruary2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Unrealized Gain (Loss) on Open Commodity Contracts (1,635,460)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMarch2015ContractsExpiringFebruary2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
% of Partners' Capital (4.39%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMarch2015ContractsExpiringFebruary2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL April 2015 contracts, expiring March 2015    
Number of Contracts 55us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClApril2015ContractsExpiringMarch2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Unrealized Gain (Loss) on Open Commodity Contracts (1,675,980)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClApril2015ContractsExpiringMarch2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
% of Partners' Capital (4.50%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClApril2015ContractsExpiringMarch2015Member
/ 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 55us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMay2015ContractsExpiringApril2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Unrealized Gain (Loss) on Open Commodity Contracts (1,705,350)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMay2015ContractsExpiringApril2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
% of Partners' Capital (4.58%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClMay2015ContractsExpiringApril2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL June 2015 contracts, expiring May 2015    
Number of Contracts 55us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJune2015ContractsExpiringMay2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Unrealized Gain (Loss) on Open Commodity Contracts (1,573,830)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJune2015ContractsExpiringMay2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
% of Partners' Capital (4.23%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= 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 55us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJuly2015ContractsExpiringJune2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Unrealized Gain (Loss) on Open Commodity Contracts (1,669,200)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJuly2015ContractsExpiringJune2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
% of Partners' Capital (4.48%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJuly2015ContractsExpiringJune2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL August 2015 contracts, expiring July 2015    
Number of Contracts 54us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClAugust2015ContractsExpiringJuly2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Unrealized Gain (Loss) on Open Commodity Contracts (1,762,850)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClAugust2015ContractsExpiringJuly2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
% of Partners' Capital (4.74%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= 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 55us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClSeptember2015ContractsExpiringAugust2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Unrealized Gain (Loss) on Open Commodity Contracts (1,595,970)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClSeptember2015ContractsExpiringAugust2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
% of Partners' Capital (4.29%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClSeptember2015ContractsExpiringAugust2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL October 2015 contracts, expiring September 2015    
Number of Contracts 55us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClOctober2015ContractsExpiringSeptember2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Unrealized Gain (Loss) on Open Commodity Contracts (1,464,030)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClOctober2015ContractsExpiringSeptember2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
% of Partners' Capital (3.93%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClOctober2015ContractsExpiringSeptember2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL November 2015 contracts, expiring October 2015    
Number of Contracts 55us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClNovember2015ContractsExpiringOctober2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Unrealized Gain (Loss) on Open Commodity Contracts (1,191,540)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClNovember2015ContractsExpiringOctober2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
% of Partners' Capital (3.20%)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 55us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClDecember2015ContractsExpiringNovember2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Unrealized Gain (Loss) on Open Commodity Contracts (868,870)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClDecember2015ContractsExpiringNovember2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
% of Partners' Capital (2.33%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClDecember2015ContractsExpiringNovember2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Open Futures Contracts, Long | United States | NYMEX WTI Crude Oil Futures CL January 2016 contracts, expiring December 2015    
Number of Contracts 55us-gaap_InvestmentOwnedBalanceContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJanuary2016ContractsExpiringDecember2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Unrealized Gain (Loss) on Open Commodity Contracts (375,650)usl_UnrealizedGainLossOnOpenCommodityFuturesContracts
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJanuary2016ContractsExpiringDecember2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
% of Partners' Capital (1.01%)usl_PercentageOfPartnersCapital
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesSeriesAxis
= usl_NymexWtiCrudeOilFuturesClJanuary2016ContractsExpiringDecember2015Member
/ us-gaap_InvestmentTypeAxis
= usl_OpenFuturesContractsLongMember		  
Cash Equivalents    
Market Value 30,998,094us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember		 59,154,742us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
% of Partners' Capital 83.25%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember		 95.25%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
Cash Equivalents | United States | US Treasury Securities    
Market Value 6,998,094us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_USTreasurySecuritiesMember		  
% of Partners' Capital 18.79%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_USTreasurySecuritiesMember		  
Cash Equivalents | United States | US Treasury Securities | U.S. Treasury Bill, 0.05%, 1/15/2015    
Principal Amount 3,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_USTreasurySecuritiesMember
/ invest_InvestmentSecuritiesSeriesAxis
= us-gaap_USTreasuryBillSecuritiesMember		  
Market Value 2,999,941us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_USTreasurySecuritiesMember
/ invest_InvestmentSecuritiesSeriesAxis
= us-gaap_USTreasuryBillSecuritiesMember		  
% of Partners' Capital 8.05%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_USTreasurySecuritiesMember
/ invest_InvestmentSecuritiesSeriesAxis
= us-gaap_USTreasuryBillSecuritiesMember		  
Cash Equivalents | United States | US Treasury Securities | U.S. Treasury Bill 0.10%, 6/25/2015    
Principal Amount 4,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_USTreasurySecuritiesMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_UsTreasuryBillSecurities1Member		  
Market Value 3,998,153us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_USTreasurySecuritiesMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_UsTreasuryBillSecurities1Member		  
% of Partners' Capital 10.74%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_USTreasurySecuritiesMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_UsTreasuryBillSecurities1Member		  
Cash Equivalents | United States | US Treasury Securities | U.S. Treasury Bill 0.7%, 3/27/2014    
Principal Amount   8,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_USTreasurySecuritiesMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_UsTreasuryBillSecurities3Member
Market Value   7,998,678us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_USTreasurySecuritiesMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_UsTreasuryBillSecurities3Member
% of Partners' Capital   12.88%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_USTreasurySecuritiesMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_UsTreasuryBillSecurities3Member
Cash Equivalents | United States | Money Market Funds    
Market Value 24,000,000us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember		 51,156,064us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
% of Partners' Capital 64.46%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember		 82.37%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
Cash Equivalents | United States | Money Market Funds | Fidelity Institutional Government Portfolio - Class I    
Principal Amount   161,900us-gaap_InvestmentOwnedBalancePrincipalAmount
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_FidelityInstitutionalGovernmentPortfolioClassIMember
Market Value   161,900us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_FidelityInstitutionalGovernmentPortfolioClassIMember
% of Partners' Capital   0.26%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_FidelityInstitutionalGovernmentPortfolioClassIMember
Cash Equivalents | United States | Money Market Funds | Goldman Sachs Financial Square Funds - Government Fund - Class FS    
Principal Amount   455,359us-gaap_InvestmentOwnedBalancePrincipalAmount
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_GoldmanSachsFinancialSquareFundsGovernmentFundClassFsMember
Market Value   455,359us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_GoldmanSachsFinancialSquareFundsGovernmentFundClassFsMember
% of Partners' Capital   0.73%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_GoldmanSachsFinancialSquareFundsGovernmentFundClassFsMember
Cash Equivalents | United States | Money Market Funds | Morgan Stanley Institutional Liquidity Funds - Government Portfolio    
Principal Amount 16,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_MorganStanleyInstitutionalLiquidityFundGovernmentPortfolioMember		 30,537,560us-gaap_InvestmentOwnedBalancePrincipalAmount
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_MorganStanleyInstitutionalLiquidityFundGovernmentPortfolioMember
Market Value 16,000,000us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_MorganStanleyInstitutionalLiquidityFundGovernmentPortfolioMember		 30,537,560us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_MorganStanleyInstitutionalLiquidityFundGovernmentPortfolioMember
% of Partners' Capital 42.97%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_MorganStanleyInstitutionalLiquidityFundGovernmentPortfolioMember		 49.17%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_MorganStanleyInstitutionalLiquidityFundGovernmentPortfolioMember
Cash Equivalents | United States | Money Market Funds | Wells Fargo Advantage Government Money Market Fund - Class I    
Principal Amount 8,000,000us-gaap_InvestmentOwnedBalancePrincipalAmount
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_WellsFargoAdvantageGovernmentMoneyMarketFundClassIMember		 20,001,245us-gaap_InvestmentOwnedBalancePrincipalAmount
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_WellsFargoAdvantageGovernmentMoneyMarketFundClassIMember
Market Value $ 8,000,000us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_WellsFargoAdvantageGovernmentMoneyMarketFundClassIMember		 $ 20,001,245us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_WellsFargoAdvantageGovernmentMoneyMarketFundClassIMember
% of Partners' Capital 21.49%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_WellsFargoAdvantageGovernmentMoneyMarketFundClassIMember		 32.21%usl_PercentageOfPartnersCapital
/ invest_InvestmentAxis
= us-gaap_CashEquivalentsMember
/ invest_InvestmentGeographicRegionAxis
= country_US
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_MoneyMarketFundsMember
/ invest_InvestmentSecuritiesSeriesAxis
= usl_WellsFargoAdvantageGovernmentMoneyMarketFundClassIMember
[1] Collateral amounted to $19,785,491 on open futures contracts.
[2] Collateral amounted to $3,548,144 on open futures contracts.