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	$ 50,061,710
Change in Unrealized Appreciation (Depreciation)	26,374		$ 2,130
Ending Value	22,283,672
Affiliated Investments [Member]
Investments In And Advances To Affiliates [Line Items]
Beginning Value	50,061,710
Ending Value	22,283,672
Affiliated Investments [Member] | Exchange-Traded Fund - Invesco Treasury Collateral ETF [Member]
Investments In And Advances To Affiliates [Line Items]
Beginning Value	20,031,492	[1]	20,012,512
Proceeds from Sales	(9,989,539)
Change in Unrealized Appreciation (Depreciation)	46,214		9,490
Realized Gain (Loss)	21,121
Ending Value	10,109,288	[1]	20,022,002
Dividend Income	$ 56,986		$ 208,173

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