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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, 2021 | Dec. 31, 2020
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | $ 533,913 | $ 632,576
Money market fund |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 76,248 | 300,339
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 | 258,141 | 531,736
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 | 76,248 | 300,339
Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 275,772 | 100,840
U.S. Treasury notes |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 179,878 | 230,088
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 | 179,878 | 230,088
Equity securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 2,015 | 1,309
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 | 2,015 | 1,309
US Government Sponsored Enterprises Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 56,012 | 9,999
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 | 56,012 | 9,999
Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 19,122 |
Municipal bonds | Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 19,122 |
Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure | 200,638 | 38,453
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 | $ 200,638 | 38,453
Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value disclosure |  | 52,388
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 |  | $ 52,388

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