Note 8 - Financial Highlights - Financial Highlights (Details) - $ / shares		3 Months Ended			6 Months Ended
		Jun. 30, 2021		Jun. 30, 2020	Jun. 30, 2021		Jun. 30, 2020
Net asset value per Share, beginning of period (in dollars per share)	[1]	$ 32.22		$ 30.74	$ 36.06	[2]	$ 29.12
Net investment loss (in dollars per share)	[1],[3]	(0.02)		(0.02)	(0.04)		(0.04)
Net realized and unrealized gain (loss) (in dollars per share)	[1],[4]	1.38		3.04	(2.44)		4.68
Net increase (decrease) in net assets from operations (in dollars per share)	[1]	1.36		3.02	(2.48)		4.64
Net asset value per Share, end of period (in dollars per share)	[1]	$ 33.58	[2]	$ 33.76	$ 33.58	[2]	$ 33.76
Total return, at net asset value	[1],[5],[6]	4.22%		9.82%	(6.88%)		15.93%
Ratio to average net assets:
Net investment loss	[1],[7]	(0.25%)		(0.25%)	(0.25%)		(0.25%)
Expenses	[1],[7]	0.25%		0.25%	0.25%		0.25%

[1] Per share amounts reflect a one-for-two reverse stock split effective at market open on May 24, 2021.

[2] Reflects a one-for-two reverse stock split effective at market open on May 24, 2021.

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

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

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

[6] Percentage is not annualized.

[7] Percentage is annualized.