Cash, Cash Equivalents and Investments - Schedule of Securities Classified as Cash and Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	$ 75,383	$ 74,598	$ 107,004	$ 107,286
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	0	0
Cash and cash equivalents, Fair Value	75,383	74,598
Short-term investments, Amortized Cost	58,640	126,296
Short-term investments, Gross Unrealized Gains	0	7
Short-term investments, Gross Unrealized Losses	(47)	(78)
Short-term investments, Fair Value	58,593	126,225
Cash equivalents and short-term investments, Amortized Cost	134,023	200,894
Cash equivalents and short-term investments, Gross Unrealized Gains	0	7
Cash equivalents and short-term investments, Gross Unrealized Losses	(47)	(78)
Cash equivalents and short-term investments, Fair Value	133,976	200,823
Cash [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	5,882	3,638
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	0	0
Cash and cash equivalents, Fair Value	5,882	3,638
Money Market Funds [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	68,651	68,561
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	0	0
Cash and cash equivalents, Fair Value	68,651	68,561
Commercial Paper (Cash Equivalents) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	850	2,399
Cash and cash equivalents, Gross Unrealized Gains	0	0
Cash and cash equivalents, Gross Unrealized Losses	0	0
Cash and cash equivalents, Fair Value	850	2,399
US Treasury Securities [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	3,994
Short-term investments, Gross Unrealized Gains	0
Short-term investments, Gross Unrealized Losses	(1)
Short-term investments, Fair Value	3,993
Corporate Bonds (Investments) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	26,853	58,464
Short-term investments, Gross Unrealized Gains	0	2
Short-term investments, Gross Unrealized Losses	(26)	(56)
Short-term investments, Fair Value	26,827	58,410
Commercial Paper (Investments) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	19,584	62,946
Short-term investments, Gross Unrealized Gains	0	5
Short-term investments, Gross Unrealized Losses	(14)	(20)
Short-term investments, Fair Value	19,570	62,931
Asset-Backed Securities [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	8,209	4,886
Short-term investments, Gross Unrealized Gains	0	0
Short-term investments, Gross Unrealized Losses	(6)	(2)
Short-term investments, Fair Value	$ 8,203	$ 4,884