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Summary of Amortized Cost and Estimated Fair Value of Available-for-sale Securities (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 148,250 $ 113,964
Estimated Fair Value 148,279 113,961
Available-for-sale Securities
   
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 123,600 82,021
Estimated Fair Value 123,629 82,020
Available-for-sale Securities | Due In One Year Or Less
   
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 76,342  
Estimated Fair Value 76,360  
Available-for-sale Securities | Due After One Year Through Two Years
   
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 47,258  
Estimated Fair Value $ 47,269