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Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale investments | $ 1,789,213 | $ 1,800,011
Cash | 6,389 | 5,812
Total cash and investments in marketable securities | 1,844,532 | 1,918,239
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 | 1,363,836 | 1,288,986
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 | 412,372 | 498,048
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 | 13,005 | 12,977
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 | 42,171 | 105,656
Certificates of Deposit [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments in marketable securities | $ 6,759 | $ 6,760

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