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Summary of Significant Accounting Policies (Additional Information) (Details)	12 Months Ended
	Dec. 31, 2014
Significant Accounting Policies [Line Items]
Minimum likelihood of tax benefits being recognized upon ultimate settlement	50.00%usl_PercentageOfIncomeTaxExaminationMinimumLikelihoodOfTaxBenefitsBeingRealizedUponSettlement
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

X

- Definition

Cash Equivalents, Maturity Period

+ References

No definition available.

+ Details

Name:	usl_CashEquivalentsMaturityPeriod
Namespace Prefix:	usl_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Number of Shares per Basket.

+ References

No definition available.

+ Details

Name:	usl_NumberOfSharesPerBasket
Namespace Prefix:	usl_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	instant

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- Details

Name:	usl_SignificantAccountingPoliciesLineItems
Namespace Prefix:	usl_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	invest_InvestmentSecuritiesClassAxis=usl_CreationBasketsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: