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FINANCIAL HIGHLIGHTS (Per Unit Performance Data and Other Supplemental Financial Data) (Details) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Per Share Operating Performance:      
Net asset value, beginning of year $ 20.39 $ 17 $ 26.59
Total income (loss) 0.82 3.52 (9.41)
Total expenses (0.16) (0.13) (0.18)
Net increase (decrease) in net asset value 0.66 3.39 (9.59)
Net asset value, end of year $ 21.05 $ 20.39 $ 17
Total Return 3.24% 19.94% (36.07%)
Ratios to Average Net Assets      
Total income (loss) 2.46% 27.09% (37.86%)
Management fees 0.60% 0.60% 0.60%
Expenses excluding management fees 0.26% 0.12% 0.20%
Net income (loss) 1.60% 26.36% (38.65%)