Cash Equivalents and Investments - Schedule of Securities Classified as Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017		Jun. 30, 2017	Dec. 31, 2016
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	$ 91,751	$ 75,383	[1]	$ 63,636	$ 74,598
Cash and cash equivalents, at Carrying Value	91,751
Cash and cash equivalents, Fair Value	91,751	75,383
Short-term investments, Amortized Cost	121,015	58,640
Short-term investments, Gross Unrealized Gains	1
Short-term investments, Gross Unrealized Losses	(36)	(47)
Short-term investments, Fair Value	120,980	58,593
Cash equivalents and short-term investments, Amortized Cost	212,766	134,023
Cash equivalents and short-term investments, Gross Unrealized Gains	1
Cash equivalents and short-term investments, Gross Unrealized Losses	(36)	(47)
Cash equivalents and short-term investments, Fair Value	212,731	133,976
Cash [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	2,932	5,882
Cash and cash equivalents, Fair Value	2,932	5,882
Money Market Funds [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	67,611	68,651
Cash and cash equivalents, Fair Value	67,611	68,651
Corporate Bonds [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	2,664
Cash and cash equivalents, Fair Value	2,664
Commercial Paper (Cash Equivalents) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Cash and cash equivalents, Amortized Cost	18,544	850
Cash and cash equivalents, Fair Value	18,544	850
U.S. Treasury Securities [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	8,459	3,994
Short-term investments, Gross Unrealized Gains	1
Short-term investments, Gross Unrealized Losses		(1)
Short-term investments, Fair Value	8,460	3,993
Corporate Bonds [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	38,788	26,853
Short-term investments, Gross Unrealized Losses	(31)	(26)
Short-term investments, Fair Value	38,757	26,827
Commercial Paper (Investments) [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	64,642	19,584
Short-term investments, Gross Unrealized Losses	(2)	(14)
Short-term investments, Fair Value	64,640	19,570
Asset-Backed Securities [Member]
Cash Cash Equivalents And Short Term Investments [Line Items]
Short-term investments, Amortized Cost	9,126	8,209
Short-term investments, Gross Unrealized Losses	(3)	(6)
Short-term investments, Fair Value	$ 9,123	$ 8,203

[1] Derived from the audited consolidated financial statements included in the Company’s Annual Report on Form 10K for the year ended December 31, 2017.