Note 2 - Significant Accounting Policies - Share Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Shares issued (in shares)	314,850,000	72,050,000	48,100,000
Shares issued	$ 3,784,339,342	$ 830,130,993	$ 590,756,513
Shares redeemed (in shares)	(167,950,000)	(99,350,000)	(50,700,000)
Distributions for Shares redeemed	$ (1,992,755,088)	$ (1,071,599,347)	$ (605,981,738)
Net increase (decrease) in Shares issued and outstanding (in shares)	146,900,000	(27,300,000)	(2,600,000)
Net increase (decrease) in net assets from capital share transactions	$ 1,791,584,254	$ (241,468,354)	$ (15,225,225)