Fair Value Measurements - Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2024 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	$ 3,192,848
Unrealized Losses within 12 months, Unrealized Losses	(8,244)
Unrealized Losses 12 months or greater, Fair Value	524,573
Unrealized Losses 12 months or greater, Unrealized Losses	(1,998)
Total, Fair Value	3,717,421
Total, Unrealized Losses	(10,242)
U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	1,445,689
Unrealized Losses within 12 months, Unrealized Losses	(2,660)
Unrealized Losses 12 months or greater, Fair Value	161,063
Unrealized Losses 12 months or greater, Unrealized Losses	(724)
Total, Fair Value	1,606,752
Total, Unrealized Losses	(3,384)
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	1,408,050
Unrealized Losses within 12 months, Unrealized Losses	(5,076)
Unrealized Losses 12 months or greater, Fair Value	184,252
Unrealized Losses 12 months or greater, Unrealized Losses	(703)
Total, Fair Value	1,592,302
Total, Unrealized Losses	(5,779)
Agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	339,109
Unrealized Losses within 12 months, Unrealized Losses	(508)
Unrealized Losses 12 months or greater, Fair Value	179,258
Unrealized Losses 12 months or greater, Unrealized Losses	(571)
Total, Fair Value	518,367
Total, Unrealized Losses	$ (1,079)