Investments in Affiliates - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Investments in and Advances to Affiliates [Line Items]
Treasury obligation measurement maximum maturity period	12 months
United States Treasury Obligations, at value	$ 226,342,665	$ 409,384,745
United States Treasury Obligations [Member]
Investments in and Advances to Affiliates [Line Items]
United States Treasury Obligations, at value	$ 226,342,665	$ 409,384,745
United States Treasury Obligations [Member] | Affiliated Investments [Member]
Investments in and Advances to Affiliates [Line Items]
Treasury obligation measurement minimum maturity period	1 month
Treasury obligation measurement maximum maturity period	12 months
Minimum [Member] | United States Treasury Obligations [Member] | Affiliated Investments [Member]
Investments in and Advances to Affiliates [Line Items]
United States Treasury Obligations, at value	$ 300,000,000