Investments - Summary of Fair Value of Available-for-Sale Investments by Type of Security (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 463,343	$ 230,351
Unrealized Gains	52	64
Unrealized Losses	(40)
Fair Value	463,355	230,415
Investments with a maturity of one year or less [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	91,195	50,993
Unrealized Gains	19	26
Fair Value	91,214	51,019
Investments with a maturity of one to two years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	372,148	179,358
Unrealized Gains	33	38
Unrealized Losses	(40)
Fair Value	372,141	179,396
U.S treasury bills [Member] | Investments with a maturity of one year or less [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	42,198	29,997
Unrealized Gains	10	21
Fair Value	42,208	30,018
U.S treasury bills [Member] | Investments with a maturity of one to two years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	191,234	3,008
Unrealized Losses	(40)
Fair Value	191,194	3,008
U.S agency securities [Member] | Investments with a maturity of one year or less [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	48,997	20,996
Unrealized Gains	9	5
Fair Value	49,006	21,001
U.S agency securities [Member] | Investments with a maturity of one to two years [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	180,914	176,350
Unrealized Gains	33	38
Fair Value	$ 180,947	$ 176,388