Summary of Significant Accounting Policies - Summary of Available -for-sale Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Amortized Cost	$ 226,443	$ 255,231
Gross Unrealized Gains	255	3
Gross Unrealized Losses		(31)
Estimated Fair Value	226,698	255,203
U.S. Treasury Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	146,552	165,959
Gross Unrealized Gains	161	2
Gross Unrealized Losses		(13)
Estimated Fair Value	146,713	165,948
Financial Institution Debt Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	53,041	65,436
Gross Unrealized Gains	69	1
Gross Unrealized Losses		(17)
Estimated Fair Value	53,110	65,420
Corporate Debt Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	18,892	23,836
Gross Unrealized Losses		(1)
Estimated Fair Value	18,892	$ 23,835
Other Asset Backed Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	7,958
Gross Unrealized Gains	25
Estimated Fair Value	$ 7,983

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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+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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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).

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-Publisher FASB
-Name Accounting Standards Codification
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