Summary of Significant Accounting Policies - Effect of Derivative Instruments on Statements of Income and Expenses (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Futures contracts, realized gain (loss)	$ (72,730,527)	$ (1,272,167,944)	$ (746,348,836)
Futures contracts, unrealized gain (loss)	452,312,410	423,877,671	(812,410,338)
Net Realized and Net Change in Unrealized Gain (Loss) on United States Treasury Obligations and Commodity Futures Contracts	379,545,648	(848,329,089)	(1,558,815,515)
Commodity Futures Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Futures contracts, realized gain (loss)	(72,730,527)	(1,272,167,944)	(746,348,836)
Futures contracts, unrealized gain (loss)	452,312,410	423,877,671	(812,410,338)
Net Realized and Net Change in Unrealized Gain (Loss) on United States Treasury Obligations and Commodity Futures Contracts	$ 379,581,883	$ (848,290,273)	$ (1,558,759,174)