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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 | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 126,535 | $ 71,360
Gross Unrealized Holding Gains | 145 | 1,252
Gross Unrealized Holding Losses | (69) | (46)
Fair Value | 126,611 | 72,566
Corporate bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 6,011 | 53,985
Gross Unrealized Holding Gains | 0 | 4
Gross Unrealized Holding Losses | (2) | (46)
Fair Value | 6,009 | 53,943
Certificate of deposits [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 110,000 |
Gross Unrealized Holding Gains | 0 |
Gross Unrealized Holding Losses | (67) |
Fair Value | 109,933 |
Bond and mutual funds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 10,399 | 17,249
Gross Unrealized Holding Gains | 47 | 1,153
Gross Unrealized Holding Losses | 0 | 0
Fair Value | 10,446 | 18,402
Equity investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 125 | 126
Gross Unrealized Holding Gains | 98 | 95
Gross Unrealized Holding Losses | 0 | 0
Fair Value | $ 223 | $ 221

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