Percentage Of Income Tax Examination Minimum Likelihood of Tax Benefits Being Realized Upon Settlement

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Name:	usl_PercentageOfIncomeTaxExaminationMinimumLikelihoodOfTaxBenefitsBeingRealizedUponSettlement
Namespace Prefix:	usl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Summary of Significant Accounting Policies (Additional Information) (Details)						3 Months Ended
	Mar. 31, 2013	Feb. 29, 2012 Creation Baskets [Member]	Feb. 29, 2012 Redemption Baskets [Member]	Feb. 29, 2012 Prior to February 29, 2012 [Member] Creation Baskets [Member]	Feb. 29, 2012 Prior to February 29, 2012 [Member] Redemption Baskets [Member]	Mar. 31, 2013 Maximum [Member]
Significant Accounting Policies [Line Items]
Minimum likelihood of tax benefits being recognized upon ultimate settlement	50.00%
Number of units per basket		50,000	50,000	100,000	100,000
Cash equivalents maturity period						6 months