Fair Value Measurements - Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2022 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	$ 1,658,315
Unrealized Losses within 12 months, Unrealized Losses	(29,147)
Unrealized Losses 12 months or greater, Fair Value	574,938
Unrealized Losses 12 months or greater, Unrealized Losses	(10,502)
Total, Fair Value	2,233,253
Total, Unrealized Losses	(39,649)
U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	877,442
Unrealized Losses within 12 months, Unrealized Losses	(13,026)
Unrealized Losses 12 months or greater, Fair Value	292,554
Unrealized Losses 12 months or greater, Unrealized Losses	(4,976)
Total, Fair Value	1,169,996
Total, Unrealized Losses	(18,002)
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	625,577
Unrealized Losses within 12 months, Unrealized Losses	(13,314)
Unrealized Losses 12 months or greater, Fair Value	258,208
Unrealized Losses 12 months or greater, Unrealized Losses	(5,035)
Total, Fair Value	883,785
Total, Unrealized Losses	(18,349)
Agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	155,296
Unrealized Losses within 12 months, Unrealized Losses	(2,807)
Unrealized Losses 12 months or greater, Fair Value	24,176
Unrealized Losses 12 months or greater, Unrealized Losses	(491)
Total, Fair Value	179,472
Total, Unrealized Losses	$ (3,298)