Investments - Available-for-sale Debt Securities in an Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	$ 483,997	$ 333,557
Less than 12 Months, Unrealized Losses	(2,741)	(3,509)
12 Months or Longer, Fair Value	241,674	372,750
12 Months or Longer, Unrealized Losses	(3,340)	(7,019)
Total, Fair Value	725,671	706,307
Total, Unrealized Losses	(6,081)	(10,528)
U.S treasury bills [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	225,573	143,089
Less than 12 Months, Unrealized Losses	(1,528)	(1,860)
12 Months or Longer, Fair Value	152,818	275,445
12 Months or Longer, Unrealized Losses	(1,864)	(4,578)
Total, Fair Value	378,391	418,534
Total, Unrealized Losses	(3,392)	(6,438)
U.S agency securities [Member]
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
Less than 12 Months, Fair Value	258,424	190,468
Less than 12 Months, Unrealized Losses	(1,213)	(1,649)
12 Months or Longer, Fair Value	88,856	97,305
12 Months or Longer, Unrealized Losses	(1,476)	(2,441)
Total, Fair Value	347,280	287,773
Total, Unrealized Losses	$ (2,689)	$ (4,090)