Investments in Affiliates - Summary of Transactions in, and Earnings from, Investments in Affiliates (Excluding Affiliated Money Market Funds) (Detail) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Investments In And Advances To Affiliates [Line Items]
Beginning Value				$ 39,094,237
Change in Unrealized Appreciation (Depreciation)	$ 92,042		$ 37,195	37,126		$ 15,291
Ending Value	31,298,707			31,298,707
Affiliated Investments [Member]
Investments In And Advances To Affiliates [Line Items]
Beginning Value				39,094,237
Ending Value	31,298,707			31,298,707
Affiliated Investments [Member] | Exchange-Traded Fund [Member]
Investments In And Advances To Affiliates [Line Items]
Beginning Value	16,052,792		16,011,779	15,990,513	[1]
Purchases at Cost						16,006,371
Change in Unrealized Appreciation (Depreciation)	65,317		28,861	127,596		34,269
Ending Value	$ 16,118,109	[1]	$ 16,040,640	$ 16,118,109	[1]	$ 16,040,640

[1] Affiliated company. The security and the Fund are affiliated by having the same investment adviser. See Note 8.