Fair Value Measurements and Investments - Summary of Investments Classified as Available-for-sale (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 40,319	$ 38,960
Gross Unrealized Gains	0	8
Gross Unrealized Losses	(39)	(27)
Fair Value	$ 40,280	$ 38,941
Available-for-sale securities, remaining maturity	3 months	2 years
U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 11,880	$ 14,992
Gross Unrealized Gains	0	3
Gross Unrealized Losses	(11)	(13)
Fair Value	11,869	14,982
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,942	7,178
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	12,942	7,178
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,991	10,326
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(27)	(13)
Fair Value	12,964	10,314
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,506	6,464
Gross Unrealized Gains	0	4
Gross Unrealized Losses	(1)	(1)
Fair Value	$ 2,505	6,467
Commercial Paper [Member] | Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents at fair value		$ 900