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 (Detail)						12 Months Ended
	Dec. 31, 2012	Feb. 29, 2012 Creation Baskets	Feb. 29, 2012 Redemption Baskets	Feb. 29, 2012 Prior to February 29, 2012 Creation Baskets	Feb. 29, 2012 Prior to February 29, 2012 Redemption Baskets	Dec. 31, 2012 Maximum
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