Investments in Affiliates - Additional Information (Detail) - USD ($)		9 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Investments In And Advances To Affiliates [Line Items]
United States Treasury Obligations, at value		$ 207,041,069	$ 244,408,079
Treasury obligation measurement maximum maturity period		12 months
United States Treasury Obligations [Member]
Investments In And Advances To Affiliates [Line Items]
United States Treasury Obligations, at value	[1]	$ 207,041,069	$ 244,408,079
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
United States Treasury Obligations [Member] | Affiliated Investments [Member] | Minimum [Member]
Investments In And Advances To Affiliates [Line Items]
United States Treasury Obligations, at value		$ 300,000,000

[1] Security may be traded on a discount basis. The interest rate shown represents the discount rate at the most recent auction date of the security prior to period end.