Summary of Significant Accounting Policies - Assets and Liabilities Measured at Fair Value (Detail) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
United States Treasury Obligations (Level 1)	$ 374,990,428	$ 282,986,507
Commodity Futures Contracts (Level 1)	21,250,208	13,354,799
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
United States Treasury Obligations (Level 1)	374,990,428	282,986,507
Commodity Futures Contracts (Level 1)	$ 21,250,208	$ 13,354,799