Note 9 - Investing in Index Futures - Effect of the Futures Contracts, by Risk Exposure Category, on the Statements of Operations (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net realized gain (loss) from futures contracts	$ (224,199,265)	$ 1,925,231
Net change in unrealized appreciation/depreciation on futures contracts	(145,592,464)	163,758,363
Commodity Contract [Member] | Net Realized Gain Loss from Futures Contracts [Member]
Net realized gain (loss) from futures contracts	(224,199,265)	1,925,231
Commodity Contract [Member] | Net Change in Unrealized Appreciation Depreciation on Futures Contracts [Member]
Net change in unrealized appreciation/depreciation on futures contracts	$ (145,592,464)	$ 163,758,363