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Summary of Significant Accounting Policies - Additional Information (Detail)
 6 Months Ended
Jun. 30, 2012
Significant Accounting Policies [Line Items]  
Minimum likelihood of tax benefits being recognized upon ultimate settlement 50.00%
Redemption Baskets
  
Significant Accounting Policies [Line Items]  
Number of units per basket 50,000
Prior to February 29, 2012 | Redemption Baskets
  
Significant Accounting Policies [Line Items]  
Number of units per basket 100,000
Maximum
  
Significant Accounting Policies [Line Items]  
Cash equivalents maturity period 6 months