Marketable Securities - Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 87,860	$ 171,632
Gross Unrealized Gains	138	20
Gross Unrealized Losses	(7)	(97)
Fair Value	87,991	171,555
U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,837	39,910
Gross Unrealized Gains	48
Gross Unrealized Losses		(12)
Fair Value	39,885	39,898
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48,023	94,999
Gross Unrealized Gains	90	20
Gross Unrealized Losses	(7)	(85)
Fair Value	$ 48,106	94,934
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		36,723
Fair Value		$ 36,723

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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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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