Investments - Available-for-sale Debt Securities in an Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 730,254	$ 677,954
Less than 12 Months, Unrealized Losses	(11,906)	(1,088)
Total, Fair Value	730,254	677,954
Total, Unrealized Losses	(11,906)	(1,088)
U.S treasury bills [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	478,880	469,386
Less than 12 Months, Unrealized Losses	(7,925)	(763)
Total, Fair Value	478,880	469,386
Total, Unrealized Losses	(7,925)	(763)
U.S agency securities [Member]
Debt Securities, Available-for-sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	251,374	208,568
Less than 12 Months, Unrealized Losses	(3,981)	(325)
Total, Fair Value	251,374	208,568
Total, Unrealized Losses	$ (3,981)	$ (325)