Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities by Major Type of Security (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 167,812	$ 83,251
Gross Unrealized Gains	5
Gross Unrealized Losses	(119)	(70)
Estimated Fair Value	167,698	83,181
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,540
Gross Unrealized Losses	(1)
Estimated Fair Value	19,539
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,860	7,799
Gross Unrealized Losses	(1)	(5)
Estimated Fair Value	17,859	7,794
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	75,999	15,919
Gross Unrealized Gains	5
Gross Unrealized Losses	(94)	(12)
Estimated Fair Value	75,910	15,907
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	50,413	19,545
Gross Unrealized Losses	(23)	(16)
Estimated Fair Value	50,390	19,529
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,000
Estimated Fair Value	$ 4,000
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		39,988
Gross Unrealized Losses		(37)
Estimated Fair Value		$ 39,951

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).

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-Publisher FASB
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-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).

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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).

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-Publisher FASB
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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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