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Summary of Amortized Cost and Estimated Fair Value of Available-for-sale Securities (Detail) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 160,229 $ 113,964
Estimated Fair Value 160,199 113,961
Available-for-sale Securities
   
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 125,699 82,021
Estimated Fair Value 125,669 82,020
Available-for-sale Securities | Due In One Year Or Less
   
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 68,935  
Estimated Fair Value 68,927  
Available-for-sale Securities | Due After One Year Through Two Years
   
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 56,764  
Estimated Fair Value $ 56,742