Derivative Instruments - Effect of Derivative Instruments on Statements of Income and Expenses (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Futures contracts, realized gain (loss)	$ 43,624,801	$ 58,338,714	$ (72,730,527)
Futures contracts, unrealized gain (loss)	(359,469,489)	14,636,898	452,312,410
Commodity Futures Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Futures contracts, realized gain (loss)	43,624,801	58,338,714	(72,730,527)
Futures contracts, unrealized gain (loss)	(359,469,489)	14,636,898	452,312,410
Net Realized and Net Change in Unrealized Gain (Loss)	$ (315,844,688)	$ 72,975,612	$ 379,581,883