Note 8 - Financial Highlights - Net Asset Value and Financial Highlights (Details) - $ / shares		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Net asset value per Share, beginning of year (in dollars per share)		$ 16.32	$ 15.62		$ 14.21
Net investment income (loss)(a) (in dollars per share)	[1]	0.15	0	[2]	(0.09)
Net realized and unrealized gain (loss)(c) (in dollars per share)	[3]	(2.48)	0.70		1.50
Net increase (decrease) in net assets from operations (in dollars per share)		(2.33)	0.70		1.41
Net asset value per Share, end of year (in dollars per share)		$ 13.99	$ 16.32		$ 15.62
Total return, at net asset value(d)	[4]	(14.28%)	4.48%		9.92%
Ratio to average net assets:
Net investment income (loss)		0.90%	(0.01%)		(0.60%)
Expenses		0.85%	0.87%		0.87%

[1] Based on average Shares outstanding during the period.

[2] Rounds to less than $0.01.

[3] The amounts reported for a Share outstanding may not accord with the change in aggregate gains and losses on investments for the period due to the timing of Trust Share transactions in relation to the fluctuating fair values of the Trust's underlying investments.

[4] Based on the change in net asset value of a Share during the period.