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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Per Share Operating Performance:    
Net asset value, beginning of period $ 17 $ 26.59
Total income (loss) 1.84 (5.99)
Total expenses (0.09) (0.15)
Net increase (decrease) in net asset value 1.75 (6.14)
Net asset value, end of period $ 18.75 $ 20.45
Total Return 10.29% (23.09%)
Ratios to Average Net Assets    
Total income (loss) 17.46% (20.93%)
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.12% 0.20%
Net income (loss) 16.91% (21.53%)
[1] Annualized.