Fair Value Measurements - Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	$ 1,267,999
Unrealized Losses within 12 months, Unrealized Losses	(3,271)
Unrealized Losses 12 months or greater, Fair Value	271,491
Unrealized Losses 12 months or greater, Unrealized Losses	(889)
Total, Fair Value	1,539,490
Total, Unrealized Losses	(4,160)
U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	317,376
Unrealized Losses within 12 months, Unrealized Losses	(1,076)
Unrealized Losses 12 months or greater, Fair Value	108,739
Unrealized Losses 12 months or greater, Unrealized Losses	(384)
Total, Fair Value	426,115
Total, Unrealized Losses	(1,460)
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	610,086
Unrealized Losses within 12 months, Unrealized Losses	(1,423)
Unrealized Losses 12 months or greater, Fair Value	141,943
Unrealized Losses 12 months or greater, Unrealized Losses	(395)
Total, Fair Value	752,029
Total, Unrealized Losses	(1,818)
Agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	340,537
Unrealized Losses within 12 months, Unrealized Losses	(772)
Unrealized Losses 12 months or greater, Fair Value	20,809
Unrealized Losses 12 months or greater, Unrealized Losses	(110)
Total, Fair Value	361,346
Total, Unrealized Losses	$ (882)