Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities by Major Type of Security (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 214,216	$ 167,812
Gross Unrealized Gains	203	5
Gross Unrealized Losses	(50)	(119)
Estimated Fair Value	214,369	167,698
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,842	19,540
Gross Unrealized Gains	27
Gross Unrealized Losses	(9)	(1)
Estimated Fair Value	12,860	19,539
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,426	17,860
Gross Unrealized Gains	12
Gross Unrealized Losses	(11)	(1)
Estimated Fair Value	22,427	17,859
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	108,108	75,999
Gross Unrealized Gains	145	5
Gross Unrealized Losses	(27)	(94)
Estimated Fair Value	108,226	75,910
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	65,340	50,413
Gross Unrealized Gains	19
Gross Unrealized Losses	(3)	(23)
Estimated Fair Value	65,356	50,390
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,500	4,000
Estimated Fair Value	$ 5,500	$ 4,000