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Summary of Cash, Cash Equivalents and Available-for-sale Marketable Securities (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Cash, cash equivalents and marketable securities [Line Items]    
Amortized Cost $ 148,250 $ 113,964
Unrealized Gains 34 9
Unrealized Losses (5) (12)
Estimated Fair Value 148,279 113,961
Cash and Cash Equivalents
   
Cash, cash equivalents and marketable securities [Line Items]    
Amortized Cost 24,650 31,943
Unrealized Losses   (2)
Estimated Fair Value 24,650 31,941
Cash and Cash Equivalents | Cash
   
Cash, cash equivalents and marketable securities [Line Items]    
Amortized Cost 2,665 2,290
Estimated Fair Value 2,665 2,290
Cash and Cash Equivalents | Money market funds
   
Cash, cash equivalents and marketable securities [Line Items]    
Amortized Cost 18,862 17,171
Estimated Fair Value 18,862 17,171
Cash and Cash Equivalents | US Government Agencies Debt Securities
   
Cash, cash equivalents and marketable securities [Line Items]    
Amortized Cost 870 9,438
Estimated Fair Value 870 9,438
Cash and Cash Equivalents | Corporate Debt Securities, Secured by U.S. Government
   
Cash, cash equivalents and marketable securities [Line Items]    
Amortized Cost 2,253 3,044
Unrealized Losses   (2)
Estimated Fair Value 2,253 3,042
Available-for-sale Securities
   
Cash, cash equivalents and marketable securities [Line Items]    
Amortized Cost 123,600 82,021
Unrealized Gains 34 9
Unrealized Losses (5) (10)
Estimated Fair Value 123,629 82,020
Available-for-sale Securities | US Government Agencies Debt Securities
   
Cash, cash equivalents and marketable securities [Line Items]    
Amortized Cost 101,274 26,488
Unrealized Gains 34  
Unrealized Losses (4) (6)
Estimated Fair Value 101,304 26,482
Available-for-sale Securities | Corporate Debt Securities, Secured by U.S. Government
   
Cash, cash equivalents and marketable securities [Line Items]    
Amortized Cost 22,326 55,533
Unrealized Gains   9
Unrealized Losses (1) (4)
Estimated Fair Value $ 22,325 $ 55,538