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Balance Sheet Components - Summary of Amortized Cost, Gross Unrealized Holding Gains or Losses, and Fair Value of Investments (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 587,885 | $ 71,360
Gross Unrealized Holding Gains | 238 | 1,252
Gross Unrealized Holding Losses | (159) | (46)
Fair Value | 587,964 | 72,566
US treasury notes and bills [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 467,296 |
Gross Unrealized Holding Gains | 109 |
Gross Unrealized Holding Losses | (69) |
Fair Value | 467,336 |
Term deposit [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 80,000 |
Gross Unrealized Holding Gains | 0 |
Gross Unrealized Holding Losses | 0 |
Fair Value | 80,000 |
Certificates of deposit [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 30,000 |
Gross Unrealized Holding Gains | 0 |
Gross Unrealized Holding Losses | (90) |
Fair Value | 29,910 |
Corporate bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost |  | 53,985
Gross Unrealized Holding Gains |  | 4
Gross Unrealized Holding Losses |  | (46)
Fair Value |  | 53,943
Bond and mutual funds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 10,464 | 17,249
Gross Unrealized Holding Gains | 20 | 1,153
Gross Unrealized Holding Losses | 0 | 0
Fair Value | 10,484 | 18,402
Equity investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 125 | 126
Gross Unrealized Holding Gains | 109 | 95
Gross Unrealized Holding Losses | 0 | 0
Fair Value | $ 234 | $ 221

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