Fair Value Measurements - Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2022 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	$ 2,284,623
Unrealized Losses within 12 months, Unrealized Losses	(35,271)
Unrealized Losses 12 months or greater, Fair Value	65,373
Unrealized Losses 12 months or greater, Unrealized Losses	(1,149)
Total, Fair Value	2,349,996
Total, Unrealized Losses	(36,420)
U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	1,082,084
Unrealized Losses within 12 months, Unrealized Losses	(16,276)
Unrealized Losses 12 months or greater, Fair Value	0
Unrealized Losses 12 months or greater, Unrealized Losses	0
Total, Fair Value	1,082,084
Total, Unrealized Losses	(16,276)
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	995,771
Unrealized Losses within 12 months, Unrealized Losses	(16,075)
Unrealized Losses 12 months or greater, Fair Value	65,373
Unrealized Losses 12 months or greater, Unrealized Losses	(1,149)
Total, Fair Value	1,061,144
Total, Unrealized Losses	(17,224)
Agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	206,768
Unrealized Losses within 12 months, Unrealized Losses	(2,920)
Unrealized Losses 12 months or greater, Fair Value	0
Unrealized Losses 12 months or greater, Unrealized Losses	0
Total, Fair Value	206,768
Total, Unrealized Losses	$ (2,920)