Note 2 - Significant Accounting Policies - Share Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Shares issued (in shares)	719,300,000	383,350,000	247,000,000
Shares issued, amount	$ 12,103,493,289	$ 5,226,042,788	$ 3,078,649,161
Shares redeemed (in shares)	(160,000,000)	(112,400,000)	(121,700,000)
Distributions for Shares redeemed	$ (2,617,761,048)	$ (1,418,257,979)	$ (1,483,295,672)
Net increase in Shares issued and outstanding (in shares)	559,300,000	270,950,000	125,300,000
Net increase in net assets from capital share transactions	$ 9,485,732,241	$ 3,807,784,809	$ 1,595,353,489