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, 2019	Dec. 31, 2018	Dec. 31, 2017
Net realized gain (loss) from futures contracts	$ 25,617,934	$ (11,940,106)	$ 3,410,884
Net change in unrealized appreciation/depreciation on futures contracts	136,157,900	(190,921,734)	77,224,746
Commodity Contract [Member] \| Net Realized Gain Loss from Futures Contracts [Member]
Net realized gain (loss) from futures contracts	25,617,934	(11,940,106)	3,410,884
Net change in unrealized appreciation/depreciation on futures contracts	$ 136,157,900	$ (190,921,734)	$ 77,224,746

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of open agreements to purchase or sell mineral resources, energy, and agricultural products at some future point held at each balance sheet date, that was included in earnings for the period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration