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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) (USD $) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 06, 2007
Per Share Operating Performance: |  |  |  |
Net asset value, beginning of year | $ 42.83 usl_NetAssetValuePerUnit | $ 39.81 usl_NetAssetValuePerUnit | $ 43.46 usl_NetAssetValuePerUnit | $ 50.00 usl_NetAssetValuePerUnit
Total income (loss) | $ (15.85)usl_TotalIncomeLossPerUnit | $ 3.43 usl_TotalIncomeLossPerUnit | $ (3.32)usl_TotalIncomeLossPerUnit |
Net expenses | $ (0.39)usl_TotalExpensesPerUnit | $ (0.41)usl_TotalExpensesPerUnit | $ (0.33)usl_TotalExpensesPerUnit |
Net increase (decrease) in net asset value | $ (16.24)usl_IncreaseDecreaseInNetAssetValuePerUnit | $ 3.02 usl_IncreaseDecreaseInNetAssetValuePerUnit | $ (3.65)usl_IncreaseDecreaseInNetAssetValuePerUnit |
Net asset value, end of year | $ 26.59 usl_NetAssetValuePerUnit | $ 42.83 usl_NetAssetValuePerUnit | $ 39.81 usl_NetAssetValuePerUnit | $ 50.00 usl_NetAssetValuePerUnit
Total Return | (37.92%)usl_RateOfReturnOnAverageAsset | 7.59%usl_RateOfReturnOnAverageAsset | (8.40%)usl_RateOfReturnOnAverageAsset |
Ratios to Average Net Assets |  |  |  |
Total income (loss) | (34.74%)usl_PercentageOfTotalRevenue | 7.54%usl_PercentageOfTotalRevenue | (6.94%)usl_PercentageOfTotalRevenue |
Management fees | 0.60%usl_ManagementIncentiveFeePercentage | 0.60%usl_ManagementIncentiveFeePercentage | 0.60%usl_ManagementIncentiveFeePercentage |
Expenses excluding management fees | 0.33%usl_ExpensesExcludingManagementFeePercentage | 0.39%usl_ExpensesExcludingManagementFeePercentage | 0.18%usl_ExpensesExcludingManagementFeePercentage |
Net income (loss) | (35.66%)usl_PercentageOfConsolidatedNetIncomeLoss | 6.55%usl_PercentageOfConsolidatedNetIncomeLoss | (7.72%)usl_PercentageOfConsolidatedNetIncomeLoss |

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