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Organization and Business (Additional Information) (Details) (USD $) | 0 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Dec. 04, 2007 | Dec. 06, 2007 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items] |  |  |  |  |  |
Fee paid by Authorized Purchasers for each order placed to create one or more Creation Baskets or to redeem one or more baskets |  |  | 350 usl_FeePaidPerOrder |  |  |
Number of initially registered units on Form S-1 with the U.S. Securities and Exchange Commission | 11,000,000 usl_NumberOfUnitsRegisteredInPeriod |  |  |  |  |
Net asset value per share |  | $ 50.00 usl_NetAssetValuePerUnit | $ 26.59 usl_NetAssetValuePerUnit | $ 42.83 usl_NetAssetValuePerUnit | $ 39.81 usl_NetAssetValuePerUnit | $ 43.46 usl_NetAssetValuePerUnit
Number of shares issued |  | 300,000 usl_UnitsIssuedDuringPeriodUnitsNewIssues |  |  |  |
Value of shares issued |  | $ 15,000,000 usl_UnitsIssuedDuringPeriodValueNewIssues |  |  |  |
Number of registered shares |  |  | 100,000,000 usl_NumberOfRegisteredUnits |  |  |
Commenced investment operations date |  | Dec. 06, 2007 |  |  |  |
Creation Baskets |  |  |  |  |  |
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items] |  |  |  |  |  |
Number of shares per basket |  |  | 50,000 usl_NumberOfUnitsPerBasket / invest_InvestmentSecuritiesClassAxis = usl_CreationBasketsMember |  |  |
Open Futures Contracts, Long | United States |  |  |  |  |  |
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items] |  |  |  |  |  |
Number of Contracts |  |  | 658 us-gaap_InvestmentOwnedBalanceContracts / invest_InvestmentGeographicRegionAxis = country_US / us-gaap_InvestmentTypeAxis = usl_OpenFuturesContractsLongMember |  |  |
Beginning on January 1, 2013 and after |  |  |  |  |  |
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items] |  |  |  |  |  |
Fee paid by Authorized Purchasers for each order placed to create one or more Creation Baskets or to redeem one or more baskets |  |  | 1,000 usl_FeePaidPerOrder / usl_PeriodAxis = usl_BeginningOnJanuary12013AndAfterMember |  |  |

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