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FINANCIAL HIGHLIGHTS (Per share Performance Data and Other Supplemental Financial Data) (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Per Share Operating Performance:
Net asset value, beginning of year	$ 12.08	$ 11.71	$ 11.02
Total income (loss)	(2.39)	0.45	0.77
Total expenses	(0.10)	(0.08)	(0.08)
Net increase (decrease) in net asset value	(2.49)	0.37	0.69
Net asset value, end of year	$ 9.59	$ 12.08	$ 11.71
Total Return	(20.61%)	3.16%	6.26%
Ratios to Average Net Assets
Total income (loss)	(15.36%)	3.75%	20.57%
Management fees	0.45%	0.45%	0.45%
Expenses excluding management fees	0.28%	0.31%	0.32%
Net income (loss)	(16.09%)	2.99%	19.80%