Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities by Major Type of Security (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 109,335	$ 83,251
Gross Unrealized Gains	29
Gross Unrealized Losses	(16)	(70)
Estimated Fair Value	109,348	83,181
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	51,428	39,988
Gross Unrealized Gains	27
Gross Unrealized Losses		(37)
Estimated Fair Value	51,455	39,951
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,996
Estimated Fair Value	2,996
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,592	7,799
Gross Unrealized Losses		(5)
Estimated Fair Value	2,592	7,794
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,933	15,919
Gross Unrealized Gains	1
Gross Unrealized Losses	(5)	(12)
Estimated Fair Value	10,929	15,907
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	41,386	19,545
Gross Unrealized Gains	1
Gross Unrealized Losses	(11)	(16)
Estimated Fair Value	$ 41,376	$ 19,529