Balance Sheet Components - Summary of Gross Unrealized Gains and Unrealized Losses of Investment Securities in Cash on Hand and Money Market Funds (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 231,344	$ 226,879
Gross Unrealized Holding Gains		1
Gross Unrealized Holding Losses	(210)	(413)
Aggregate Fair Value	231,134	226,467
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	119,692	61,167
Gross Unrealized Holding Losses	(90)	(81)
Aggregate Fair Value	119,602	61,086
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	111,652	165,712
Gross Unrealized Holding Gains		1
Gross Unrealized Holding Losses	(120)	(332)
Aggregate Fair Value	$ 111,532	$ 165,381

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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)
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
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