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FINANCIAL HIGHLIGHTS - Per share performance data and other supplemental financial data (Details) - $ / shares
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Per Share Operating Performance:      
Net asset value, beginning of period $ 24.35 $ 23.34 [1] $ 37.18 [1]
Total income (loss) (7.16) 1.30 (13.49)
Total expenses (0.28) (0.29) (0.35)
Net increase (decrease) in net asset value (7.44) 1.01 (13.84)
Net asset value, end of period $ 16.91 $ 24.35 $ 23.34 [1]
Total Return (30.55%) 4.33% (37.22%)
Ratios to Average Net Assets      
Total income (loss) (24.25%) 30.51% (37.75%)
Management fees 0.60% 0.60% 0.60%
Expenses excluding management fees 0.73% 0.68% 0.70%
Net income (loss) (25.58%) 29.23% (39.05%)
[1] On January 4, 2018, there was a 1for4 reverse share split. The Statement of Changes in Partners’ Capital have been adjusted for the period shown to reflect the 1for4 reverse share split on a retroactive basis.