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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Per Share Operating Performance:    
Net asset value, beginning of period $ 17 $ 26.59
Total income (loss) 2.4 0.36
Total expenses (0.06) (0.11)
Net increase (decrease) in net asset value 2.34 0.25
Net asset value, end of period $ 19.34 $ 26.84
Total Return 13.76% 0.94%
Ratios to Average Net Assets    
Total income (loss) 20.77% 5.13%
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.13% 0.22%
Net income (loss) 20.40% 4.72%
[1] Annualized