Financial Instruments - Summary of Estimated Fair Value and Related Valuation Input Hierarchy of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Total Fair Value	$ 433,837	$ 499,345
Fair Value, Measurements, Recurring
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	433,676	499,049
Total Unrealized Gain	195	311
Total Unrealized Loss	(34)	(15)
Total Fair Value	433,837	499,345
Fair Value, Measurements, Recurring | Money Market Funds | Level 1
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	138,100	168,343
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	138,100	168,343
Fair Value, Measurements, Recurring | U.S. Treasury Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	176,010	230,239
Total Unrealized Gain	72	113
Total Unrealized Loss	0	(6)
Total Fair Value	176,082	230,346
Fair Value, Measurements, Recurring | Government Agency Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	25,218	22,537
Total Unrealized Gain	15	22
Total Unrealized Loss	(6)	(3)
Total Fair Value	25,227	22,556
Fair Value, Measurements, Recurring | Corporate Debt Obligations | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	61,259	50,080
Total Unrealized Gain	100	166
Total Unrealized Loss	(20)	(1)
Total Fair Value	61,339	50,245
Fair Value, Measurements, Recurring | Commercial Paper | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	22,242	17,990
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	22,242	17,990
Fair Value, Measurements, Recurring | Asset-Backed Securities | Level 2
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	10,847	9,860
Total Unrealized Gain	8	10
Total Unrealized Loss	(8)	(5)
Total Fair Value	10,847	9,865
Fair Value, Measurements, Recurring | Amounts Classified As Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	(149,773)	(199,090)
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Fair Value	(149,773)	(199,090)
Fair Value, Measurements, Recurring | Amounts Classified As Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Total Amortized Cost	283,903	299,959
Total Unrealized Gain	195	311
Total Unrealized Loss	(34)	(15)
Total Fair Value	$ 284,064	$ 300,255