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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Per Share Operating Performance:    
Net asset value, beginning of period $ 21.05 $ 20.39
Total income (loss) 1.64 (1.83)
Total expenses (0.04) (0.04)
Net increase (decrease) in net asset value 1.6 (1.87)
Net asset value, end of period $ 22.65 $ 18.52
Total Return 7.60% (9.17%)
Ratios to Average Net Assets    
Total income (loss) 7.71% (9.31%)
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.21% 0.29%
Net income (loss) 7.51% (9.53%)
[1] Annualized.