Balance Sheet Components - Summary of Gross Unrealized Gains and Unrealized Losses of Investment Securities Excluding Investments in Money Market Funds (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 236,718	$ 226,879
Gross Unrealized Holding Gains	7	1
Gross Unrealized Holding Losses	(770)	(413)
Aggregate Fair Value	235,955	226,467
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	70,036	61,167
Gross Unrealized Holding Gains	7
Gross Unrealized Holding Losses	(84)	(81)
Aggregate Fair Value	69,959	61,086
Corporate Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	166,682	165,712
Gross Unrealized Holding Gains		1
Gross Unrealized Holding Losses	(686)	(332)
Aggregate Fair Value	$ 165,996	$ 165,381

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
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-Publisher FASB
-Name Accounting Standards Codification
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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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-Publisher FASB
-Name Accounting Standards Codification
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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