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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Per Share Operating Performance:    
Net asset value, beginning of year $ 10.26 $ 9.26
Total income (loss) 0.26 0.05
Net expenses (0.02) (0.02)
Net increase (decrease) in net asset value 0.24 0.03
Net asset value, end of year $ 10.50 $ 9.29
Total Return 2.34% 0.32%
Ratios to Average Net Assets    
Total income (loss) 2.52% 0.88%
Management fees [1] 0.75% 0.75%
Total expenses excluding management fees [1] 1.88% 1.08%
Expenses waived [1] (1.73%) (0.93%)
Net expenses excluding management fees [1] 0.15% 0.15%
Net income (loss) 2.30% 0.65%
[1] Annualized.