Financial Instruments - Summary of Estimated Fair Value and Related Valuation Input Hierarchy of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Total Fair Value	$ 187,101	$ 310,584
Fair Value, Measurements, Recurring
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	186,928	310,924
Total Unrealized Gain	185	20
Total Unrealized Loss	(12)	(360)
Total Fair Value	187,101	310,584
Fair Value, Measurements, Recurring | Money Market Funds | Level 1
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	36,621	38,708
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	36,621	38,708
Fair Value, Measurements, Recurring | U.S. Treasury Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	69,324	111,164
Total Unrealized Gain	49	4
Total Unrealized Loss	(1)	(80)
Total Fair Value	69,372	111,088
Fair Value, Measurements, Recurring | Government Agency Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	9,079	15,206
Total Unrealized Gain	1	1
Total Unrealized Loss	(4)	(32)
Total Fair Value	9,076	15,175
Fair Value, Measurements, Recurring | Corporate Debt Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	63,788	121,017
Total Unrealized Gain	135	15
Total Unrealized Loss	(6)	(217)
Total Fair Value	63,917	120,815
Fair Value, Measurements, Recurring | Commercial Paper | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	2,846	12,935
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	2,846	12,935
Fair Value, Measurements, Recurring | Asset-Backed Securities | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	5,270	11,894
Total Unrealized Gain	0	0
Total Unrealized Loss	(1)	(31)
Total Fair Value	5,269	11,863
Fair Value, Measurements, Recurring | Amounts Classified As Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	(56,124)	(61,651)
Total Unrealized Gain	(1)	0
Total Unrealized Loss	0	0
Total Fair Value	(56,125)	(61,651)
Fair Value, Measurements, Recurring | Amounts Classified As Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	130,804	249,273
Total Unrealized Gain	184	20
Total Unrealized Loss	(12)	(360)
Total Fair Value	$ 130,976	$ 248,933