Accounting Policies - Derivative Fair Value of Assets and Liabilities Using Unobservable Inputs (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	$ 25,318,863	$ 35,614,486
Liabilities	209,000	699,200
Net fair value	25,109,863	34,915,286
Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	211,200	621,000
Liabilities	209,000	699,200
Net fair value	2,200	(78,200)
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	25,107,663	34,993,486
Net fair value	25,107,663	34,993,486
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	25,107,663	34,993,486
U.S. Government Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	25,107,663	34,993,486
Options Purchased Indices [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	211,200	621,000
Options Purchased Indices [Member] | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	211,200	621,000
Written Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	209,000	699,200
Written Options [Member] | Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	$ 209,000	$ 699,200