Note 9 - Investing in Index Futures (Details) - Effect of the Futures Contracts, by Risk Exposure Category, on the Statements of Operations - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Net realized gain (loss) from futures contracts	$ (152,499,032)	$ (99,275,900)	$ (222,057,163)	$ (138,661,150)
Net change in unrealized appreciation/depreciation on futures contracts	(15,131,979)	(43,732,303)	52,483,833	49,609,765
Net Realized Gain Loss from Futures Contracts [Member] | Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net realized gain (loss) from futures contracts	(152,499,032)	(99,275,900)	(222,057,163)	(138,661,150)
Net Change In Unrealized Appreciation Depreciation On Futures Contracts [Member] | Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net change in unrealized appreciation/depreciation on futures contracts	$ (15,131,979)	$ (43,732,303)	$ 52,483,833	$ 49,609,765