Investments in Marketable Securities - Summary of Investments Classified As Available-For-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 273,273	$ 277,156
Gross Unrealized Gains	261	1
Gross Unrealized Losses	(21)	(416)
Aggregate Estimated Fair Value	273,513	276,741
Certificate of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	251
Aggregate Estimated Fair Value	251
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		3,479
Aggregate Estimated Fair Value		3,479
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	273,022	273,677
Gross Unrealized Gains	261	1
Gross Unrealized Losses	(21)	(416)
Aggregate Estimated Fair Value	$ 273,262	$ 273,262

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)
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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).

+ References

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-Publisher FASB
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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

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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), classified as current.

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