Cash, Cash Equivalents and Investments - Amortized Cost to Fair Value (Detail) $ in Thousands	Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 492,675
Gross Unrealized Gains	5,690
Fair Value, Total	498,365
Treasury Bills [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	131,861
Treasury Bills [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	34,978
Gross Unrealized Gains	425
Gross Unrealized Losses	0
Fair Value, Total	35,403
Treasury Bills [Member] \| Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	95,522
Gross Unrealized Gains	936
Gross Unrealized Losses	0
Fair Value, Total	96,458
Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	92,218
Asset-Backed Securities [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	24,999
Gross Unrealized Gains	161
Gross Unrealized Losses	0
Fair Value, Total	25,160
Asset-Backed Securities [Member] \| Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	66,329
Gross Unrealized Gains	729
Gross Unrealized Losses	0
Fair Value, Total	67,058
Corporate Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	236,142
Corporate Bonds [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,394
Gross Unrealized Gains	320
Gross Unrealized Losses	0
Fair Value, Total	32,714
Corporate Bonds [Member] \| Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	200,549
Gross Unrealized Gains	2,879
Gross Unrealized Losses	0
Fair Value, Total	203,428
Sovereign Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	12,100
Sovereign Bonds [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,590
Gross Unrealized Gains	70
Gross Unrealized Losses	0
Fair Value, Total	8,660
Sovereign Bonds [Member] \| Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,379
Gross Unrealized Gains	61
Gross Unrealized Losses	0
Fair Value, Total	3,440
Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	7,038
Municipal Bonds [Member] \| Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,297
Gross Unrealized Gains	6
Gross Unrealized Losses	0
Fair Value, Total	1,303
Municipal Bonds [Member] \| Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,725
Gross Unrealized Gains	10
Gross Unrealized Losses
Fair Value, Total	5,735
Agency Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value, Total	19,006
Agency Bonds [Member] \| Long-term investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	18,913
Gross Unrealized Gains	93
Gross Unrealized Losses	0
Fair Value, Total	$ 19,006

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

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-Publisher FASB
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-Topic 320
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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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