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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 $ 9.26 $ 11.75
Total income (loss) 0.05 (1.08)
Net expenses (0.02) (0.02)
Net increase (decrease) in net asset value 0.03 (1.1)
Net asset value, end of period $ 9.29 $ 10.65
Total Return 0.32% (9.36%)
Ratios to Average Net Assets    
Total income (loss) 0.88% (13.70%)
Management fees [1] 0.75% 0.75%
Total expenses excluding management fees [1] 1.08% 0.59%
Expenses waived [1] (0.93%) (0.44%)
Net expenses excluding management fees [1] 0.15% 0.15%
Net income (loss) 0.65% (13.92%)
[1] Annualized.