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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) |  |  | 9 Months Ended
--- | --- | --- | ---
Sep. 30, 2012 | Feb. 29, 2012 Redemption Baskets | Feb. 29, 2012 Prior to February 29, 2012 Redemption Baskets | Sep. 30, 2012 Maximum
Significant Accounting Policies [Line Items] |  |  |  |
Basis points subtracted from overnight Federal Funds Rate | 0.32% |  |  |
Minimum likelihood of tax benefits being recognized upon ultimate settlement | 50.00% |  |  |
Number of units per basket |  | $ 50,000 | $ 100,000 |
Cash equivalents maturity period |  |  |  | 6 months

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- Details
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