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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, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 467,850	$ 587,885
Gross Unrealized Holding Gains	784	238
Gross Unrealized Holding Losses	(25)	(159)
Fair Value	468,609	587,964
US treasury notes and bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	426,995	467,296
Gross Unrealized Holding Gains	536	109
Gross Unrealized Holding Losses	(25)	(69)
Fair Value	427,506	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	32	0
Gross Unrealized Holding Losses	0	(90)
Fair Value	30,032	29,910
Bond and mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,730	10,464
Gross Unrealized Holding Gains	86	20
Gross Unrealized Holding Losses	0	0
Fair Value	10,816	10,484
Equity investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	125	125
Gross Unrealized Holding Gains	130	109
Gross Unrealized Holding Losses	0	0
Fair Value	$ 255	$ 234

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- Definition

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+ 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

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