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FINANCIAL HIGHLIGHTS - Per share performance data and other supplemental financial data (Details) - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Per Share Operating Performance:
Net asset value, beginning of year	$ 9.20	$ 16.91	$ 24.35
Total income (loss)	3.33	(7.55)	(7.16)
Total expenses	(0.14)	(0.16)	(0.28)
Net increase (decrease) in net asset value	3.19	(7.71)	(7.44)
Net asset value, end of period	$ 12.39	$ 9.20	$ 16.91
Total Return	34.67%	(45.59%)	(30.55%)
Ratios to Average Net Assets
Total income (loss)	6.56%	(49.07%)	(24.25%)
Management fees	0.60%	0.60%	0.60%
Expenses excluding management fees	0.51%	0.75%	0.73%
Net income (loss)	5.45%	(50.42%)	(25.58%)