Note 9 - Investing in Index Futures - Effect of the Futures Contracts, by Risk Exposure Category, on the Statements of Operations (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net realized gain (loss) from futures contracts	$ (11,940,106)	$ 3,410,884	$ 76,363,644
Net change in unrealized appreciation/depreciation on futures contracts	(190,921,734)	77,224,746	12,329,480
Commodity Contract [Member] | Net Realized Gain Loss from Futures Contracts [Member]
Net realized gain (loss) from futures contracts	(11,940,106)	3,410,884	76,363,644
Net change in unrealized appreciation/depreciation on futures contracts	0	0	0
Commodity Contract [Member] | Net Change in Unrealized Appreciation Depreciation on Futures Contracts [Member]
Net realized gain (loss) from futures contracts	0	0	0
Net change in unrealized appreciation/depreciation on futures contracts	$ (190,921,734)	$ 77,224,746	$ 12,329,480