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)	120,750,000	64,000,000	55,800,000
Shares issued	$ 1,836,353,140	$ 954,051,929	$ 873,170,446
Shares redeemed (in shares)	(70,500,000)	(65,900,000)	(75,950,000)
Distributions for Shares redeemed	$ (1,101,564,648)	$ (982,882,736)	$ (1,226,578,286)
Net increase (decrease) in Shares issued and outstanding (in shares)	50,250,000	(1,900,000)	(20,150,000)
Net increase (decrease) in net assets from capital share transactions	$ 734,788,492	$ (28,830,807)	$ (353,407,840)