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Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($) $ in Thousands | Sep. 30, 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 | $ 1,646,043 | $ 1,800,011
Cash | 28,166 | 5,812
Total cash and investments in marketable securities | 1,726,754 | 1,918,239
Corporate Notes and Bonds | Fair Value Hierarchy Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale investments | 1,163,575 | 1,288,986
Corporate Commercial Paper | Fair Value Hierarchy Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale investments | 482,468 | 498,048
Obligations of U.S. Government Agencies | Fair Value Hierarchy Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale investments | 0 | 12,977
Money Market Funds | Fair Value Hierarchy Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments in marketable securities | 45,674 | 105,656
Certificates of Deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments in marketable securities | $ 6,871 | $ 6,760

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