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v2.4.1.9

Summary of Significant Accounting Policies (Additional Information) (Details) | 12 Months Ended
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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 | 50,000 usl_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: |