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FINANCIAL HIGHLIGHTS - Per share performance data and other supplemental financial data (Details) - $ / shares
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Per Share Operating Performance:    
Net asset value, beginning of period $ 16.91 $ 24.35
Total income (loss) (4.43) (0.85)
Total expenses (0.05) (0.08)
Net increase (decrease) in net asset value (4.48) (0.93)
Net asset value, end of period $ 12.43 $ 23.42
Total Return (26.49%) (3.82%)
Ratios to Average Net Assets    
Total income (loss) (28.48%) 0.11%
Management fees [1] 0.60% 0.60%
Expenses excluding management fees [1] 0.73% 0.78%
Net income (loss) (28.81%) (0.23%)
[1] Annualized.