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Organization and Business - Additional Information (Detail) (USD $) | 1 Months Ended |  |  |  |  |  |  |
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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 |  |  |  |  |  |  |  |

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