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Balance Sheet Components - Summary of Amortized Cost, Gross Unrealized Holding Gains or Losses, and Fair Value of Available-for-Sale Investments (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 8,272 | $ 467,850
Gross Unrealized Holding Gains | 119 | 784
Gross Unrealized Holding Losses | (3) | (25)
Fair Value | 8,388 | 468,609
U.S. treasury notes and bills [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost |  | 426,995
Gross Unrealized Holding Gains |  | 536
Gross Unrealized Holding Losses |  | (25)
Fair Value |  | 427,506
Certificate of deposit [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost |  | 30,000
Gross Unrealized Holding Gains |  | 32
Gross Unrealized Holding Losses |  | 0
Fair Value |  | 30,032
Bond and mutual funds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 8,147 | 10,730
Gross Unrealized Holding Gains | 0 | 86
Gross Unrealized Holding Losses | (3) | 0
Fair Value | 8,144 | 10,816
Equity investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 125 | 125
Gross Unrealized Holding Gains | 119 | 130
Gross Unrealized Holding Losses | 0 | 0
Fair Value | $ 244 | $ 255

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