Investments in Affiliates - Summary of Transactions in, and Earnings from, Investments in Affiliates (Excluding Affiliated Money Market Funds) (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2020		Mar. 31, 2019
Investments in and Advances to Affiliates [Line Items]
Beginning Value	$ 200,021,832
Change in Unrealized Appreciation (Depreciation)	270,857		$ 13,601
Ending Value	144,775,016
Affiliated Investments [Member]
Investments in and Advances to Affiliates [Line Items]
Beginning Value	200,021,832
Ending Value	144,775,016
Affiliated Investments [Member] | Exchange-Traded Fund [Member]
Investments in and Advances to Affiliates [Line Items]
Beginning Value	79,123,338	[1]	79,048,368
Change in Unrealized Appreciation (Depreciation)	487,305		37,485
Ending Value	79,610,643	[1]	79,085,853
Dividend Income	$ 329,786		$ 456,738

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