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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Per Share Operating Performance:    
Net asset value, beginning of period $ 9.29 $ 8.69
Total income (loss) (2.46) 0 [1]
Total expenses (0.05) (0.04)
Net increase (decrease) in net asset value (2.51) (0.04)
Net asset value, end of period $ 6.78 $ 8.65
Total Return (27.02%) (0.46%)
Ratios to Average Net Assets    
Total income (loss) (29.11%) 7.65%
Management fees [2] 0.60% 0.60%
Expenses excluding management fees [2] 0.73% 0.58%
Net income (loss) (29.77%) 7.06%
[1] Amount less than $0.005.
[2] Annualized.