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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Per Share Operating Performance:    
Net asset value, beginning of period $ 13.69 $ 18.32
Total income (loss) (0.81) 1.33
Net expenses (0.06) (0.09)
Net decrease in net asset value (0.87) 1.24
Net asset value, end of period $ 12.82 $ 19.56
Total Return (6.36%) 6.77%
Ratios to Average Net Assets    
Total income (loss) (3.57%) 10.27%
Management fees [1] 0.75% 0.75%
Total expenses excluding management fees [1] 0.62% 0.95%
Expenses waived [1] (0.47%) (0.74%)
Net expenses excluding management fees [1] 0.15% 0.21%
Net income (loss) (4.01%) 9.79%
[1] Annualized