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FINANCIAL HIGHLIGHTS - Per share performance data and other supplemental financial data (Details) - $ / shares
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Per Share Operating Performance:    
Net asset value, beginning of year $ 9.20 $ 16.91
Total income (loss) 0.44 (4.43)
Total expenses (0.03) (0.05)
Net increase (decrease) in net asset value 0.41 (4.48)
Net asset value, end of period $ 9.61 $ 12.43
Total Return 4.46% (26.49%)
Ratios to Average Net Assets    
Total income (loss) 8.46% (28.48%)
Management fees [1] 0.60% 0.60%
Total expenses excluding management fees [1] 0.57% 0.73%
Net income (loss) 8.18% (28.81%)
[1] Annualized