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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	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 411,363	$ 467,850
Gross Unrealized Holding Gains	2,729	784
Gross Unrealized Holding Losses	0	(25)
Fair Value	414,092	468,609
U.S. treasury notes and bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	381,238	426,995
Gross Unrealized Holding Gains	2,516	536
Gross Unrealized Holding Losses	0	(25)
Fair Value	383,754	427,506
Certificate of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,000	30,000
Gross Unrealized Holding Gains	115	32
Gross Unrealized Holding Losses	0	0
Fair Value	30,115	30,032
Bond and mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		10,730
Gross Unrealized Holding Gains		86
Gross Unrealized Holding Losses		0
Fair Value		10,816
Equity investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	125	125
Gross Unrealized Holding Gains	98	130
Gross Unrealized Holding Losses	0	0
Fair Value	$ 223	$ 255

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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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

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

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