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Organization and Business - Additional Information (Detail) (USD $)
 1 Months Ended
Dec. 06, 2007
Dec. 04, 2007
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Feb. 29, 2012
Creation Baskets
Feb. 29, 2012
Prior to February 29, 2012
Creation Baskets
Dec. 31, 2012
Beginning on January 1, 2013 and after
Dec. 31, 2012
Open Cleared Swap and Futures Contracts
Open Futures Contracts, Long
United States
Contract
Dec. 31, 2011
Open Cleared Swap and Futures Contracts
Open Futures Contracts, Long
United States
Contract
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]                      
Number of Contracts                   1,068 1,716
Number of units per basket             50,000 100,000      
Fee paid by Authorized Purchasers for each order placed to create one or more Creation Baskets or to redeem one or more baskets     350           1,000    
Number of initially registered units on Form S-1 with the U.S. Securities and Exchange Commission   11,000,000                  
Net asset value per unit $ 50.00   $ 39.81 $ 43.46 $ 42.91 $ 40.37          
Number of units issued 300,000                    
Value of units issued $ 15,000,000                    
Commenced investment operations date Dec. 06, 2007                    
Number of registered units     111,000,000