Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities by Major Type of Security (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 141,879	$ 167,812
Gross Unrealized Gains	85	5
Gross Unrealized Losses	(12)	(119)
Estimated Fair Value	141,952	167,698
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,921	19,540
Gross Unrealized Gains	16
Gross Unrealized Losses		(1)
Estimated Fair Value	15,937	19,539
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,440	17,860
Gross Unrealized Gains	7
Gross Unrealized Losses		(1)
Estimated Fair Value	12,447	17,859
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	69,914	75,999
Gross Unrealized Gains	56	5
Gross Unrealized Losses	(10)	(94)
Estimated Fair Value	69,960	75,910
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	38,104	50,413
Gross Unrealized Gains	6
Gross Unrealized Losses	(2)	(23)
Estimated Fair Value	38,108	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