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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, 2019 | Dec. 31, 2018
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 442,170 | $ 587,885
Gross Unrealized Holding Gains | 894 | 238
Gross Unrealized Holding Losses | (25) | (159)
Fair Value | 443,039 | 587,964
US treasury notes and bills [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 401,377 | 467,296
Gross Unrealized Holding Gains | 688 | 109
Gross Unrealized Holding Losses | (25) | (69)
Fair Value | 402,040 | 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
Certificate of deposit [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 30,000 | 30,000
Gross Unrealized Holding Gains | 0 | 0
Gross Unrealized Holding Losses | 0 | (90)
Fair Value | 30,000 | 29,910
Bond and mutual funds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 10,668 | 10,464
Gross Unrealized Holding Gains | 80 | 20
Gross Unrealized Holding Losses | 0 | 0
Fair Value | 10,748 | 10,484
Equity investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 125 | 125
Gross Unrealized Holding Gains | 126 | 109
Gross Unrealized Holding Losses | 0 | 0
Fair Value | $ 251 | $ 234

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as 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=118939692&loc=d3e27161-111563 + Details
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