Financial Instruments - Summary of Estimated Fair Value and Related Valuation Input Hierarchy of Available-for-Sale Securities (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	$ 196,508	$ 245,841
Total Unrealized Gain	12	20
Total Unrealized Loss	(100)	(204)
Total Fair Value	196,420	245,657
Money Market Funds | Level 1
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	47,593	28,816
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	47,593	28,816
U.S. Treasury Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	51,692	65,403
Total Unrealized Gain	2	3
Total Unrealized Loss	(16)	(21)
Total Fair Value	51,678	65,385
Government Agency Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	4,760	23,860
Total Unrealized Gain	1	5
Total Unrealized Loss	(6)	(5)
Total Fair Value	4,755	23,860
Corporate Debt Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	82,402	113,649
Total Unrealized Gain	9	8
Total Unrealized Loss	(69)	(172)
Total Fair Value	82,342	113,485
Commercial Paper | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	2,085	699
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	2,085	699
Asset-Backed Securities | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	7,976	13,414
Total Unrealized Gain	0	4
Total Unrealized Loss	(9)	(6)
Total Fair Value	7,967	13,412
Amounts Classified As Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	56,692	37,944
Total Unrealized Gain	0	0
Total Unrealized Loss	0	(1)
Total Fair Value	56,692	37,943
Amounts Classified As Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	139,816	207,897
Total Unrealized Gain	12	20
Total Unrealized Loss	(100)	(203)
Total Fair Value	$ 139,728	$ 207,714