Cash Equivalents, Restricted Cash and Investments - Amortized Cost and Fair Value of Cash Equivalents and Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 398,207	$ 458,098
Gross Unrealized Gains	29	121
Gross Unrealized Losses	(110)	(7)
Available-for-sale Securities	398,126	458,212
Money Market Fund [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	72,699	30,139
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Available-for-sale Securities	72,699	30,139
U.S. Treasury Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	62,601	176,625
Gross Unrealized Gains	6	60
Gross Unrealized Losses	(1)	0
Available-for-sale Securities	62,606	176,685
Non US Government Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	11,847
Gross Unrealized Gains	0
Gross Unrealized Losses	(10)
Available-for-sale Securities	11,837
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	86,910	140,364
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Available-for-sale Securities	86,910	140,364
Asset-backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	39,266	25,706
Gross Unrealized Gains	3	23
Gross Unrealized Losses	(19)	0
Available-for-sale Securities	39,250	25,729
Corporate Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	124,884	72,764
Gross Unrealized Gains	20	38
Gross Unrealized Losses	(80)	(7)
Available-for-sale Securities	$ 124,824	72,795
Government-Sponsored Agencies [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		12,500
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Available-for-sale Securities		$ 12,500

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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