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				$ 317,746,227
Change in Unrealized Appreciation (Depreciation)	$ 603,611		$ 266,321	259,962		$ 150,092
Ending Value	399,907,435			399,907,435
Affiliated Investments [Member]
Investments In And Advances To Affiliates [Line Items]
Beginning Value				317,746,227
Ending Value	399,907,435			399,907,435
Affiliated Investments [Member] | Exchange-Traded Fund [Member]
Investments In And Advances To Affiliates [Line Items]
Beginning Value	250,715,232		250,074,684	249,742,548	[1]
Purchases at Cost						249,990,226
Change in Unrealized Appreciation (Depreciation)	1,020,132		450,756	1,992,816		535,214
Ending Value	$ 251,735,364	[1]	$ 250,525,440	$ 251,735,364	[1]	$ 250,525,440

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