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Investment Securities, Available-for-Sale and Classified as Cash Equivalents (Detail) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Schedule of Investments [Line Items]    
Amortized Cost $ 197,971 $ 56,382
Unrealized Gains 78 5
Unrealized (Losses) (108) (5)
Estimated Fair Value 197,941 56,382
U.S. Treasury notes
   
Schedule of Investments [Line Items]    
Amortized Cost   2,029
Unrealized Gains   1
Estimated Fair Value   2,030
Government sponsored enterprise securities
   
Schedule of Investments [Line Items]    
Amortized Cost 95,490 54,353
Unrealized Gains 4 4
Unrealized (Losses) (17) (5)
Estimated Fair Value 95,477 54,352
Commercial paper
   
Schedule of Investments [Line Items]    
Amortized Cost 40,509  
Unrealized Gains 74  
Estimated Fair Value 40,583  
Corporate debt securities
   
Schedule of Investments [Line Items]    
Amortized Cost 61,972  
Unrealized (Losses) (91)  
Estimated Fair Value $ 61,881