Summary of Significant Accounting Policies - Effect of Derivative Instruments on Statements of Income and Expenses (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Futures contracts, realized gain (loss)	$ (66,087,932)	$ (301,640,755)	$ (265,281,304)	$ (727,075,762)
Futures contracts, unrealized gain (loss)	381,444,063	468,319,830	570,979,786	647,818,126
Net Realized and Net Change in Unrealized Gain (Loss) on United States Treasury Obligations and Commodity Futures Contracts	315,552,721	166,693,354	306,073,813	(79,271,144)
Commodity Futures Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Futures contracts, realized gain (loss)	(66,087,932)	(301,640,755)	(265,281,304)	(727,075,762)
Futures contracts, unrealized gain (loss)	381,444,063	468,319,830	570,979,786	647,818,126
Net Realized and Net Change in Unrealized Gain (Loss) on United States Treasury Obligations and Commodity Futures Contracts	$ 315,356,131	$ 166,679,075	$ 305,698,482	$ (79,257,636)