Fair Value Measurements - Derivative Instruments, Fair Value Based on Hierarchy (Detail) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative assets	$ 159,521,113	$ 215,122,195
Derivative liabilities	108,396,235	121,642,142
Net fair value	51,124,878	93,480,053
Futures And Exchange-Cleared Swaps [Member]
Derivative [Line Items]
Derivative assets	62,232,394	54,683,502
Derivative liabilities	75,408,741	84,404,729
Options Purchased [Member]
Derivative [Line Items]
Derivative assets	97,288,719	160,438,693
Options Premium Received [Member]
Derivative [Line Items]
Derivative liabilities	32,987,494	37,237,413
Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member]
Derivative [Line Items]
Derivative assets	159,521,113	215,122,195
Derivative liabilities	108,396,235	121,642,142
Net fair value	51,124,878	93,480,053
Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member] | Futures And Exchange-Cleared Swaps [Member]
Derivative [Line Items]
Derivative assets	62,232,394	54,683,502
Derivative liabilities	75,408,741	84,404,729
Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member] | Options Purchased [Member]
Derivative [Line Items]
Derivative assets	97,288,719	160,438,693
Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member] | Options Premium Received [Member]
Derivative [Line Items]
Derivative liabilities	32,987,494	37,237,413
Significant Other Observable Inputs (Level 2) [Member]
Derivative [Line Items]
Derivative assets
Derivative liabilities
Net fair value
Significant Other Observable Inputs (Level 2) [Member] | Futures And Exchange-Cleared Swaps [Member]
Derivative [Line Items]
Derivative assets
Derivative liabilities
Significant Other Observable Inputs (Level 2) [Member] | Options Purchased [Member]
Derivative [Line Items]
Derivative assets
Significant Other Observable Inputs (Level 2) [Member] | Options Premium Received [Member]
Derivative [Line Items]
Derivative liabilities
Significant Unobservable Inputs (Level 3) [Member]
Derivative [Line Items]
Derivative assets
Derivative liabilities
Net fair value
Significant Unobservable Inputs (Level 3) [Member] | Futures And Exchange-Cleared Swaps [Member]
Derivative [Line Items]
Derivative assets
Derivative liabilities
Significant Unobservable Inputs (Level 3) [Member] | Options Purchased [Member]
Derivative [Line Items]
Derivative assets
Significant Unobservable Inputs (Level 3) [Member] | Options Premium Received [Member]
Derivative [Line Items]
Derivative liabilities