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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Per Share Operating Performance:      
Net asset value, beginning of year $ 9.26 $ 11.75 $ 9.72
Total income (loss) 1.09 (2.40) 2.12
Net expenses (0.09) (0.09) (0.09)
Net increase (decrease) in net asset value 1.00 (2.49) 2.03
Net asset value, end of year $ 10.26 $ 9.26 $ 11.75
Total Return 10.80% (21.19%) 20.88%
Ratios to Average Net Assets      
Total income (loss) 9.73% (29.05%) 21.94%
Management fees 0.75% 0.75% 0.75%
Total expenses excluding management fees [1] 1.08% 0.74% 0.90%
Expenses waived (0.93%) (0.59%) (0.75%)
Net expenses excluding management fees 0.15% 0.15% 0.15%
Net income (loss) 8.83% (29.95%) 21.04%
[1] Total returns are calculated based on the change in value during the period. An individual shareholder’s total return and ratio may vary from the above total returns and ratios based on the timing of contributions to and withdrawals from UNL.