Investments in Affiliates -Summary of Transactions in, and Earnings from, Investments in Affiliates (Excluding Affiliated Money Market Fund (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2020		Mar. 31, 2019
Investments in and Advances to Affiliates [Line Items]
Beginning Value	$ 105,630,007
Change in Unrealized Appreciation (Depreciation)	89,994		$ 7,523
Ending Value	845,180,412
Affiliated Investments [Member]
Investments in and Advances to Affiliates [Line Items]
Beginning Value	105,630,007
Ending Value	845,180,412
Affiliated Investments [Member] | Exchange-Traded Fund [Member]
Investments in and Advances to Affiliates [Line Items]
Beginning Value	79,893,780	[1]	79,818,080
Change in Unrealized Appreciation (Depreciation)	492,050		37,850
Ending Value	80,385,830	[1]	79,855,930
Dividend Income	$ 332,997		$ 461,186

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