Note 2 - Significant Accounting Policies - Share Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares issued (in shares)	228,250,000	314,850,000	72,050,000
Shares issued, amount	$ 2,782,113,163	$ 3,784,339,342	$ 830,130,993
Shares redeemed (in shares)	(68,350,000)	(167,950,000)	(99,350,000)
Distributions for Shares redeemed	$ (826,600,408)	$ (1,992,755,088)	$ (1,071,599,347)
Net increase (decrease) in Shares issued and outstanding (in shares)	159,900,000	146,900,000	(27,300,000)
Net increase (decrease) in net assets from capital share transactions	$ 1,955,512,755	$ 1,791,584,254	$ (241,468,354)