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FINANCIAL HIGHLIGHTS - Per share performance data and other supplemental financial data (Details) - $ / shares
 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Per Share Operating Performance:    
Net asset value, beginning of period $ 24.35 $ 23.34 [1]
Total income (loss) (4.20) 1.73
Total expenses (0.22) (0.21)
Net increase (decrease) in net asset value (4.42) 1.52
Net asset value, end of period $ 19.93 $ 24.86
Total Return (18.15%) 6.51%
Ratios to Average Net Assets    
Total income (loss) (7.66%) 14.28%
Management fees [2] 0.60% 0.60%
Expenses excluding management fees [2] 0.74% 0.70%
Net income (loss) (8.66%) 13.31%
[1] On January 4, 2018, there was a 1for4 reverse share split. Historical shares outstanding, net asset value per share and market value per share have been adjusted for the period shown to reflect the 1for4 reverse share split on a retroactive basis.
[2] Annualized.