Financial Instruments - Net Unrealized Gain (Losses) on Open Contracts and Longest Contract Maturities (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Net unrealized gain (loss) on open contracts	$ 342,570	$ 1,293,243
Exchange-Traded [Member]
Investment Holdings [Line Items]
Net unrealized gain (loss) on open contracts	$ 318,779	$ 774,574
Longest maturity date	Dec. 31, 2016	Mar. 31, 2016
Off-Exchange-Traded [Member]
Investment Holdings [Line Items]
Net unrealized gain (loss) on open contracts	$ 23,791	$ 518,669
Longest maturity date	Dec. 31, 2015	Mar. 31, 2015