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FINANCIAL HIGHLIGHTS (Per share Performance Data and Other Supplemental Financial Data) (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Per Share Operating Performance:
Net asset value, beginning of year	$ 11.71	$ 11.02	$ 20.15
Total income (loss)	0.45	0.77	(9.01)
Total expenses	(0.08)	(0.08)	(0.12)
Net increase (decrease) in net asset value	0.37	0.69	(9.13)
Net asset value, end of year	$ 12.08	$ 11.71	$ 11.02
Total Return	3.16%	6.26%	(45.31%)
Ratios to Average Net Assets
Total income (loss)	3.75%	20.57%	(49.08%)
Management fees	0.45%	0.45%	0.45%
Expenses excluding management fees	0.31%	0.32%	0.29%
Net income (loss)	2.99%	19.80%	(49.82%)