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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. 31, 2010 | Nov. 18, 2009
Per Unit Operating Performance: |  |  |  |  |
Net asset value, beginning of year | $ 18.32 unl_NetAssetValuePerUnit | $ 17.23 unl_NetAssetValuePerUnit | $ 21.21 unl_NetAssetValuePerUnit | $ 21.21 unl_NetAssetValuePerUnit | $ 50.00 unl_NetAssetValuePerUnit
Total income (loss) | $ (4.33)unl_TotalIncomeLossPerUnit | $ 1.32 unl_TotalIncomeLossPerUnit | $ (3.78)unl_TotalIncomeLossPerUnit |  |
Net expenses | $ (0.30)unl_TotalExpensesPerUnit | $ (0.23)unl_TotalExpensesPerUnit | $ (0.20)unl_TotalExpensesPerUnit |  |
Net increase (decrease) in net asset value | $ (4.63)unl_IncreaseDecreaseInNetAssetValuePerUnit | $ 1.09 unl_IncreaseDecreaseInNetAssetValuePerUnit | $ (3.98)unl_IncreaseDecreaseInNetAssetValuePerUnit |  |
Net asset value, end of year | $ 13.69 unl_NetAssetValuePerUnit | $ 18.32 unl_NetAssetValuePerUnit | $ 17.23 unl_NetAssetValuePerUnit | $ 21.21 unl_NetAssetValuePerUnit | $ 50.00 unl_NetAssetValuePerUnit
Total Return | (25.27%)unl_RateOfReturnOnAverageAsset | 6.33%unl_RateOfReturnOnAverageAsset | (18.76%)unl_RateOfReturnOnAverageAsset |  |
Ratios to Average Net Assets |  |  |  |  |
Total income (loss) | (19.76%)unl_PercentageOfTotalRevenue | 13.23%unl_PercentageOfTotalRevenue | (11.80%)unl_PercentageOfTotalRevenue |  |
Management fees | 0.75%unl_ManagementFee | 0.75%unl_ManagementFee | 0.75%unl_ManagementFee |  |
Total expenses excluding management fees | 0.85%unl_ExpensesExcludingManagementFeePercentage | 0.51%unl_ExpensesExcludingManagementFeePercentage | 0.40%unl_ExpensesExcludingManagementFeePercentage |  |
Expenses waived | (0.67%)unl_PercentageWaiverExpense | (0.30%)unl_PercentageWaiverExpense | (0.18%)unl_PercentageWaiverExpense |  |
Net expenses excluding management fees | 0.18%unl_ExpenseExcludingManagementFeePercentageNet | 0.21%unl_ExpenseExcludingManagementFeePercentageNet | 0.22%unl_ExpenseExcludingManagementFeePercentageNet |  |
Net income (loss) | (20.70%)unl_PercentageOfConsolidatedNetIncome | 12.27%unl_PercentageOfConsolidatedNetIncome | (12.77%)unl_PercentageOfConsolidatedNetIncome |  |

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