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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 $ 18.32unl_NetAssetValuePerUnit $ 17.23unl_NetAssetValuePerUnit $ 21.21unl_NetAssetValuePerUnit $ 21.21unl_NetAssetValuePerUnit $ 50.00unl_NetAssetValuePerUnit
Total income (loss) $ (4.33)unl_TotalIncomeLossPerUnit $ 1.32unl_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 $ 1.09unl_IncreaseDecreaseInNetAssetValuePerUnit $ (3.98)unl_IncreaseDecreaseInNetAssetValuePerUnit    
Net asset value, end of year $ 13.69unl_NetAssetValuePerUnit $ 18.32unl_NetAssetValuePerUnit $ 17.23unl_NetAssetValuePerUnit $ 21.21unl_NetAssetValuePerUnit $ 50.00unl_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