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Summary of Significant Accounting Policies (Additional Information) (Details)
 3 Months Ended
Mar. 31, 2015
Significant Accounting Policies [Line Items]  
Minimum likelihood of tax benefits being recognized upon ultimate settlement 50.00%usl_PercentageOfIncomeTaxExaminationMinimumLikelihoodOfTaxBenefitsBeingRealizedUponSettlement
Fee paid by Authorized Purchasers for each order placed to create one or more Creation Baskets or to redeem one or more baskets 350usl_FeePaidPerOrder
Creation Baskets  
Significant Accounting Policies [Line Items]  
Number of Shares per basket 50,000usl_NumberOfSharesPerBasket
/ invest_InvestmentSecuritiesClassAxis
= usl_CreationBasketsMember
Maximum  
Significant Accounting Policies [Line Items]  
Cash equivalents maturity period 6 months