Derivative Instruments - Effect of Derivative Instruments on Statements of Income and Expenses (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Futures contracts, realized gain (loss)	$ 91,323,528	$ (63,372,537)	$ 149,343,521	$ (1,182,345)
Futures contracts, unrealized gain (loss)	17,443,048	(42,197,634)	(3,075,603)	(201,066,322)
Net Realized and Net Change in Unrealized Gain (Loss) on United States Treasury Obligations, Affiliated Investments and Commodity Futures Contracts	110,380,326	(104,865,769)	148,515,132	(201,605,700)
Commodity Futures Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Futures contracts, realized gain (loss)	91,323,528	(63,372,537)	149,343,521	(1,182,345)
Futures contracts, unrealized gain (loss)	17,443,048	(42,197,634)	(3,075,603)	(201,066,322)
Net Realized and Net Change in Unrealized Gain (Loss) on United States Treasury Obligations, Affiliated Investments and Commodity Futures Contracts	$ 108,766,576	$ (105,570,171)	$ 146,267,918	$ (202,248,667)