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FINANCIAL HIGHLIGHTS - Per share performance data and other supplemental financial data (Details) - $ / shares
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Per Share Operating Performance:        
Net asset value, beginning of period $ 10.25 $ 19.83 $ 16.91 $ 24.35
Total income (loss) 1.14 0.16 (5.43) (4.20)
Total expenses (0.04) (0.06) (0.13) (0.22)
Net increase (decrease) in net asset value 1.10 0.10 (5.56) (4.42)
Net asset value, end of period $ 11.35 $ 19.93 $ 11.35 $ 19.93
Total Return 10.73% 0.50% (32.88%) (18.15%)
Ratios to Average Net Assets        
Total income (loss) 14.92% 5.62% (36.50%) (7.66%)
Management fees [1] 0.60% 0.60% 0.60% 0.60%
Expenses excluding management fees [1] 0.73% 0.67% 0.81% 0.74%
Net income (loss) 14.59% 5.30% (37.56%) (8.66%)
[1] Annualized.