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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) (USD $) | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Sep. 30, 2013 | Nov. 18, 2009
Per Unit Operating Performance: |  |  |  |  |  |
Net asset value, beginning of period | $ 13.69 unl_NetAssetValuePerUnit |  | $ 18.32 unl_NetAssetValuePerUnit |  |  | $ 50.00 unl_NetAssetValuePerUnit
Total income (loss) | $ (1.06)unl_TotalIncomeLossPerUnit |  | $ 1.36 unl_TotalIncomeLossPerUnit |  |  |
Net expenses | $ (0.03)unl_TotalExpensesPerUnit |  | $ (0.05)unl_TotalExpensesPerUnit |  |  |
Net decrease in net asset value | $ (1.09)unl_IncreaseDecreaseInNetAssetValuePerUnit |  | $ 1.31 unl_IncreaseDecreaseInNetAssetValuePerUnit |  |  |
Net asset value, end of period | $ 12.60 unl_NetAssetValuePerUnit |  | $ 19.63 unl_NetAssetValuePerUnit |  |  | $ 50.00 unl_NetAssetValuePerUnit
Total Return | (7.96%)unl_RateOfReturnOnAverageAsset |  |  |  | 7.15%unl_RateOfReturnOnAverageAsset |
Ratios to Average Net Assets |  |  |  |  |  |
Total income (loss) | (7.42%)unl_PercentageOfTotalRevenue |  | 8.92%unl_PercentageOfTotalRevenue |  |  |
Management fees | 0.75%unl_ManagementIncentiveFeePercentage | [1] | 0.75%unl_ManagementIncentiveFeePercentage | [1] |  |
Total expenses excluding management fees | 0.57%unl_ExpensesExcludingManagementFeePercentage | [1] | 0.87%unl_ExpensesExcludingManagementFeePercentage | [1] |  |
Expenses waived | (0.41%)unl_PercentageWaiverExpense | [1] | (0.64%)unl_PercentageWaiverExpense | [1] |  |
Net expenses excluding management fees | 0.16%unl_ExpenseExcludingManagementFeePercentageNet | [1] | 0.23%unl_ExpenseExcludingManagementFeePercentageNet | [1] |  |
Net income (loss) | (7.65%)unl_PercentageOfConsolidatedNetIncome |  | 8.68%unl_PercentageOfConsolidatedNetIncome |  |  |
[1] | Annualized

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