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Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale investments | $ 347,326 | $ 330,713
Cash | 4,460 | 4,355
Total cash and investments in marketable securities | 353,220 | 389,102
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 | 128,096 | 160,920
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 | 216,253 | 156,044
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 | 2,977 | 13,749
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 | 302 | 51,104
Certificates of Deposit [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments in marketable securities | $ 1,132 | $ 2,930

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