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Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($) $ in Thousands | Jun. 30, 2020 | Dec. 31, 2019
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities, available-for-sale, accumulated gross unrealized gain, before tax |  | $ 2,100
Debt securities, available-for-sale accumulated gross unrealized loss, before tax |  | (300)
Cash | $ 6,071 | 5,829
Total cash and investments in marketable securities | 1,210,563 | 1,603,981
Fair Value Hierarchy Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities, available-for-sale, amortized cost | 1,139,707 |
Debt securities, available-for-sale, accumulated gross unrealized gain, before tax | 4,570 |
Debt securities, available-for-sale accumulated gross unrealized loss, before tax | (170) |
Available-for-sale investments | 1,144,107 | 1,507,655
Corporate notes and bonds | Fair Value Hierarchy Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities, available-for-sale, amortized cost | 893,901 |
Debt securities, available-for-sale, accumulated gross unrealized gain, before tax | 4,095 |
Debt securities, available-for-sale accumulated gross unrealized loss, before tax | (143) |
Available-for-sale investments | 897,853 | 1,132,182
Corporate commercial paper | Fair Value Hierarchy Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities, available-for-sale, amortized cost | 235,811 |
Debt securities, available-for-sale, accumulated gross unrealized gain, before tax | 475 |
Debt securities, available-for-sale accumulated gross unrealized loss, before tax | (26) |
Available-for-sale investments | 236,260 | 375,473
Obligations of U.S. government agencies | Fair Value Hierarchy Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Debt securities, available-for-sale, amortized cost | 9,995 |
Debt securities, available-for-sale, accumulated gross unrealized gain, before tax | 0 |
Debt securities, available-for-sale accumulated gross unrealized loss, before tax | (1) |
Available-for-sale investments | 9,994 | 0
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 | 52,135 | 83,546
Certificates of Deposit |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments in marketable securities | $ 8,250 | $ 6,951

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