Summary of Significant Accounting Policies - Summary of Available -for-sale Marketable Securities (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Marketable Securities [Line Items]
Amortized Cost	$ 255,231
Gross Unrealized Gains	3
Gross Unrealized Losses	(31)
Estimated Fair Value	255,203
U.S. Treasury Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	165,959
Gross Unrealized Gains	2
Gross Unrealized Losses	(13)
Estimated Fair Value	165,948
Financial Institution Debt Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	65,436
Gross Unrealized Gains	1
Gross Unrealized Losses	(17)
Estimated Fair Value	65,420
Corporate Debt Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	23,836
Gross Unrealized Losses	(1)
Estimated Fair Value	$ 23,835

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)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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
-Subparagraph (a)
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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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