Note 2 - Significant Accounting Policies - Share Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares issued (in shares)	64,000,000	55,800,000	102,750,000
Shares issued	$ 954,051,929	$ 873,170,446	$ 1,702,628,324
Shares redeemed (in shares)	(65,900,000)	(75,950,000)	(76,400,000)
Distributions for Shares redeemed	$ (982,882,736)	$ (1,226,578,286)	$ (1,236,224,053)
Net increase (decrease) in Shares issued and outstanding (in shares)	(1,900,000)	(20,150,000)	26,350,000
Net increase (decrease) in net assets from capital share transactions	$ (28,830,807)	$ (353,407,840)	$ 466,404,271