Fair Value Measurements - Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2023 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	$ 814,619
Unrealized Losses within 12 months, Unrealized Losses	(4,204)
Unrealized Losses 12 months or greater, Fair Value	871,880
Unrealized Losses 12 months or greater, Unrealized Losses	(13,804)
Total, Fair Value	1,686,499
Total, Unrealized Losses	(18,008)
U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	274,507
Unrealized Losses within 12 months, Unrealized Losses	(1,144)
Unrealized Losses 12 months or greater, Fair Value	439,448
Unrealized Losses 12 months or greater, Unrealized Losses	(6,826)
Total, Fair Value	713,955
Total, Unrealized Losses	(7,970)
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	457,708
Unrealized Losses within 12 months, Unrealized Losses	(2,677)
Unrealized Losses 12 months or greater, Fair Value	370,215
Unrealized Losses 12 months or greater, Unrealized Losses	(5,623)
Total, Fair Value	827,923
Total, Unrealized Losses	(8,300)
Agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	82,404
Unrealized Losses within 12 months, Unrealized Losses	(383)
Unrealized Losses 12 months or greater, Fair Value	62,217
Unrealized Losses 12 months or greater, Unrealized Losses	(1,355)
Total, Fair Value	144,621
Total, Unrealized Losses	$ (1,738)