Fair Value Measurements - Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2023 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	$ 1,632,547
Unrealized Losses within 12 months, Unrealized Losses	(11,123)
Unrealized Losses 12 months or greater, Fair Value	610,052
Unrealized Losses 12 months or greater, Unrealized Losses	(9,182)
Total, Fair Value	2,242,599
Total, Unrealized Losses	(20,305)
U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	457,325
Unrealized Losses within 12 months, Unrealized Losses	(3,346)
Unrealized Losses 12 months or greater, Fair Value	291,908
Unrealized Losses 12 months or greater, Unrealized Losses	(4,145)
Total, Fair Value	749,233
Total, Unrealized Losses	(7,491)
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	743,043
Unrealized Losses within 12 months, Unrealized Losses	(4,714)
Unrealized Losses 12 months or greater, Fair Value	256,414
Unrealized Losses 12 months or greater, Unrealized Losses	(3,817)
Total, Fair Value	999,457
Total, Unrealized Losses	(8,531)
Agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	432,179
Unrealized Losses within 12 months, Unrealized Losses	(3,063)
Unrealized Losses 12 months or greater, Fair Value	61,730
Unrealized Losses 12 months or greater, Unrealized Losses	(1,220)
Total, Fair Value	493,909
Total, Unrealized Losses	$ (4,283)