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Fair Value Measurements - Fair Value Measurements of Cash Equivalents, Available-For-Sale Investment Securities and Equity Securities (Detail) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | $ 632,160 | $ 623,123
Money market fund |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 439,026 | 98,084
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 514,209 | 328,994
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market fund |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 439,026 | 98,084
Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 117,951 | 294,129
U.S. Treasury notes |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 74,138 | 228,603
U.S. Treasury notes | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 74,138 | 228,603
Equity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 1,045 | 2,307
Equity securities | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 1,045 | 2,307
US Government Sponsored Enterprises Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 14,666 | 103,878
US Government Sponsored Enterprises Debt Securities [Member] | Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 14,666 | 103,878
Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 80,799 | 88,039
Corporate debt securities | Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 80,799 | 88,039
Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 22,486 | 102,212
Commercial paper | Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | $ 22,486 | $ 102,212

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