Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities by Major Type of Security (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 62,758	$ 83,251
Gross Unrealized Losses	(114)	(70)
Estimated Fair Value	62,644	83,181
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,263	39,988
Gross Unrealized Losses	(87)	(37)
Estimated Fair Value	29,176	39,951
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,799	7,799
Gross Unrealized Losses	(3)	(5)
Estimated Fair Value	3,796	7,794
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,351	15,919
Gross Unrealized Losses	(11)	(12)
Estimated Fair Value	9,340	15,907
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,345	19,545
Gross Unrealized Losses	(13)	(16)
Estimated Fair Value	$ 20,332	$ 19,529