Trading Activities - Summary of Transactions in Options Written (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Contract
Summary Of Investment Holdings [Line Items]
Outstanding options, Premiums Received, beginning of year	$ 635,950
Outstanding options, Premiums Received, end of year	$ 157,300
Calls [Member]
Summary Of Investment Holdings [Line Items]
Outstanding options, Contracts, beginning of year | Contract	920
Options written, Contracts | Contract	28,262
Options expired, Contracts | Contract	(26,950)
Options closed, Contracts | Contract	(1,792)
Outstanding options, Contracts, end of year | Contract	440
Outstanding options, Notional, beginning of year	$ 548,550,000
Options written, Notional	17,908,253,000
Options expired, Notional	(17,109,586,000)
Options closed, Notional	(1,142,850,000)
Outstanding options, Notional, end of year	204,367,000
Outstanding options, Premiums Received, beginning of year	57,500
Options written, Premiums Received	2,016,813
Options expired, Premiums Received	(1,920,188)
Options closed, Premiums Received	(137,625)
Outstanding options, Premiums Received, end of year	$ 16,500
Puts [Member]
Summary Of Investment Holdings [Line Items]
Outstanding options, Contracts, beginning of year | Contract	920
Options written, Contracts | Contract	93,303
Options expired, Contracts | Contract	(76,099)
Options closed, Contracts | Contract	(17,068)
Outstanding options, Contracts, end of year | Contract	1,056
Outstanding options, Notional, beginning of year	$ 480,470,000
Options written, Notional	54,565,785,000
Options expired, Notional	(44,518,737,000)
Options closed, Notional	(9,867,518,000)
Outstanding options, Notional, end of year	660,000,000
Outstanding options, Premiums Received, beginning of year	578,450
Options written, Premiums Received	28,098,738
Options expired, Premiums Received	(22,548,450)
Options closed, Premiums Received	(5,987,938)
Outstanding options, Premiums Received, end of year	$ 140,800