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)	247,000,000	228,250,000	314,850,000
Shares issued, amount	$ 3,078,649,161	$ 2,782,113,163	$ 3,784,339,342
Shares redeemed (in shares)	(121,700,000)	(68,350,000)	(167,950,000)
Distributions for Shares redeemed	$ (1,483,295,672)	$ (826,600,408)	$ (1,992,755,088)
Net increase in Shares issued and outstanding (in shares)	125,300,000	159,900,000	146,900,000
Net increase in net assets from capital share transactions	$ 1,595,353,489	$ 1,955,512,755	$ 1,791,584,254