Fair Value Measurements - Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2023 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	$ 2,155,949
Unrealized Losses within 12 months, Unrealized Losses	(12,711)
Unrealized Losses 12 months or greater, Fair Value	407,787
Unrealized Losses 12 months or greater, Unrealized Losses	(4,073)
Total, Fair Value	2,563,736
Total, Unrealized Losses	(16,784)
U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	688,395
Unrealized Losses within 12 months, Unrealized Losses	(3,625)
Unrealized Losses 12 months or greater, Fair Value	182,156
Unrealized Losses 12 months or greater, Unrealized Losses	(1,702)
Total, Fair Value	870,551
Total, Unrealized Losses	(5,327)
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	925,541
Unrealized Losses within 12 months, Unrealized Losses	(5,928)
Unrealized Losses 12 months or greater, Fair Value	163,152
Unrealized Losses 12 months or greater, Unrealized Losses	(1,789)
Total, Fair Value	1,088,693
Total, Unrealized Losses	(7,717)
Agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	542,013
Unrealized Losses within 12 months, Unrealized Losses	(3,158)
Unrealized Losses 12 months or greater, Fair Value	62,479
Unrealized Losses 12 months or greater, Unrealized Losses	(582)
Total, Fair Value	604,492
Total, Unrealized Losses	$ (3,740)