Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities by Major Type of Security (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 116,970	$ 167,812
Gross Unrealized Gains	171	5
Gross Unrealized Losses	(6)	(119)
Estimated Fair Value	117,135	167,698
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,992	19,540
Gross Unrealized Gains	32
Gross Unrealized Losses		(1)
Estimated Fair Value	13,024	19,539
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,447	17,860
Gross Unrealized Gains	16
Gross Unrealized Losses		(1)
Estimated Fair Value	12,463	17,859
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	62,529	75,999
Gross Unrealized Gains	109	5
Gross Unrealized Losses	(6)	(94)
Estimated Fair Value	62,632	75,910
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,502	50,413
Gross Unrealized Gains	14
Gross Unrealized Losses		(23)
Estimated Fair Value	23,516	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