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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Per Share Operating Performance:    
Net asset value, beginning of period [1] $ 23.34 $ 37.18
Total income (loss) 1.73 (10.73) [1]
Total expenses (0.21) (0.27) [1]
Net increase (decrease) in net asset value 1.52 (11.00) [1]
Net asset value, end of period $ 24.86 $ 26.18 [1]
Total Return 6.51% (29.60%)
Ratios to Average Net Assets    
Total income (loss) 14.28% (31.58%)
Management fees [2] 0.60% 0.60%
Expenses excluding management fees [2] 0.70% 0.68%
Net income (loss) 13.31% (32.54%)
[1] On January 4, 2018, there was a 1-for-4 reverse share split. Historical shares outstanding, net asset value per share and market value per share have been adjusted to reflect the 1-for-4 reverse share split on a retroactive basis.
[2] Annualized.