Note 2 - Significant Accounting Policies - Share Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Shares issued (in shares)	383,350,000	247,000,000	228,250,000
Shares issued, amount	$ 5,226,042,788	$ 3,078,649,161	$ 2,782,113,163
Shares redeemed (in shares)	(112,400,000)	(121,700,000)	(68,350,000)
Distributions for Shares redeemed	$ (1,418,257,979)	$ (1,483,295,672)	$ (826,600,408)
Net increase in Shares issued and outstanding (in shares)	270,950,000	125,300,000	159,900,000
Net increase (decrease) in net assets from capital share transactions	$ 3,807,784,809	$ 1,595,353,489	$ 1,955,512,755