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Investments - Summary of Fair Value of Available-for-Sale Investments by Type of Security (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Schedule Of Held To Maturity Securities [Line Items] |  |
Amortized Cost | $ 230,351 |
Unrealized Gains | 64 |
Fair Value | 230,415 |
Investments with a maturity of one year or less [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Amortized Cost | 50,993 | $ 313,537
Unrealized Gains | 26 | 325
Fair Value | 51,019 | 313,862
Investments with a maturity of one to two years [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Amortized Cost | 179,358 |
Unrealized Gains | 38 |
Fair Value | 179,396 |
U.S treasury bills [Member] | Investments with a maturity of one year or less [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Amortized Cost | 29,997 | 232,336
Unrealized Gains | 21 | 268
Fair Value | 30,018 | 232,604
U.S treasury bills [Member] | Investments with a maturity of one to two years [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Amortized Cost | 3,008 |
Fair Value | 3,008 |
U.S agency securities [Member] | Investments with a maturity of one year or less [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Amortized Cost | 20,996 | 81,201
Unrealized Gains | 5 | 57
Fair Value | 21,001 | $ 81,258
U.S agency securities [Member] | Investments with a maturity of one to two years [Member] |  |
Schedule Of Held To Maturity Securities [Line Items] |  |
Amortized Cost | 176,350 |
Unrealized Gains | 38 |
Fair Value | $ 176,388 |

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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