Note 10 - Investment Valuation - Valuation of the Trust's Investments by the Fair Value Hierarchy (Details) - USD ($)		Mar. 31, 2020	Dec. 31, 2019
Futures [Member]
Investment, fair value	[1]	$ (125,957,813)	$ 19,634,651
US Treasury Bill Securities [Member]
Investment, fair value		491,322,390	750,519,365
Fair Value, Inputs, Level 1 [Member] | Futures [Member]
Investment, fair value	[1]	(125,957,813)	19,634,651
Fair Value, Inputs, Level 1 [Member] | US Treasury Bill Securities [Member]
Investment, fair value		0	0
Fair Value, Inputs, Level 2 [Member] | Futures [Member]
Investment, fair value	[1]	0	0
Fair Value, Inputs, Level 2 [Member] | US Treasury Bill Securities [Member]
Investment, fair value		491,322,390	750,519,365
Fair Value, Inputs, Level 3 [Member] | Futures [Member]
Investment, fair value	[1]	$ 0	0
Fair Value, Inputs, Level 3 [Member] | US Treasury Bill Securities [Member]
Investment, fair value			$ 0

[1] Shown at the unrealized appreciation (depreciation) on the contracts.