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Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($)
$ in Thousands
Jun. 30, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale investments $ 216,248 $ 250,444
Cash, including restricted cash 1,044 1,842
Total cash and investments in marketable securities 274,854 308,944
Corporate Notes and Bonds [Member] | Fair Value Hierarchy Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale investments 121,333 181,969
Corporate Commercial Paper [Member] | Fair Value Hierarchy Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale investments 78,052 61,150
Obligations of U.S. government agencies [Member] | Fair Value Hierarchy Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale investments 16,863 7,325
Money Market Funds [Member] | Fair Value Hierarchy Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments in marketable securities 54,632 53,728
Certificates of Deposit [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments in marketable securities $ 2,930 $ 2,930