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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 $ 42.83usl_NetAssetValuePerUnit $ 39.81usl_NetAssetValuePerUnit $ 43.46usl_NetAssetValuePerUnit $ 50.00usl_NetAssetValuePerUnit
Total income (loss) $ (15.85)usl_TotalIncomeLossPerUnit $ 3.43usl_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 $ 3.02usl_IncreaseDecreaseInNetAssetValuePerUnit $ (3.65)usl_IncreaseDecreaseInNetAssetValuePerUnit  
Net asset value, end of year $ 26.59usl_NetAssetValuePerUnit $ 42.83usl_NetAssetValuePerUnit $ 39.81usl_NetAssetValuePerUnit $ 50.00usl_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