Note 10 - Investment Valuation - Valuation of the Trust's Investments by the Fair Value Hierarchy (Details) - USD ($)		Dec. 31, 2018	Dec. 31, 2017
Futures [Member]
Investment, fair value	[1]	$ (116,523,249)	$ 74,398,485
US Treasury Bill Securities [Member]
Investment, fair value		1,199,726,652	1,343,654,642
Fair Value, Inputs, Level 1 [Member] | Futures [Member]
Investment, fair value	[1]	(116,523,249)	74,398,485
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		1,199,726,652	1,343,654,642
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	$ 0

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