Note 10 - Investment Valuation (Details) - Valuation of the Investments by the Fair Value Hierarchy - USD ($)		Sep. 30, 2015	Dec. 31, 2014
Futures Contracts [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, fair value	[1]	$ (16,884,863)	$ (69,368,696)
US Treasury Bill Securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, fair value		827,006,587	784,772,217
Fair Value, Inputs, Level 1 [Member] | Futures Contracts [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, fair value	[1]	(16,884,863)	(69,368,696)
Fair Value, Inputs, Level 2 [Member] | US Treasury Bill Securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, fair value		$ 827,006,587	$ 784,772,217

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