Fair Value Measurements - Summary of Gross Unrealized Gains and Losses of Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,421,310	$ 438,052
Gross Unrealized Gains	123	451
Gross Unrealized Losses	(429)	(36)
Fair Value	1,421,004	438,467
U.S. Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	302,080	160,589
Gross Unrealized Gains	8	11
Gross Unrealized Losses	(66)	(12)
Fair Value	302,022	160,588
U.S. Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	561,120	122,882
Gross Unrealized Gains	69	240
Gross Unrealized Losses	(244)	(15)
Fair Value	560,945	123,107
International Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	238,376	57,485
Gross Unrealized Gains	42	200
Gross Unrealized Losses	(102)	(9)
Fair Value	238,316	57,676
U.S. Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	120,133	45,963
Gross Unrealized Gains	2
Gross Unrealized Losses	(6)
Fair Value	120,129	45,963
International Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	198,601	51,133
Gross Unrealized Gains	2
Gross Unrealized Losses	(11)
Fair Value	198,592	$ 51,133
U.S. Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,000
Fair Value	$ 1,000

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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Amount, before tax, of unrealized loss 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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Amortized cost of 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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