Fair Value Measurements - Unrealized Loss Position (Details) $ in Millions	Jun. 30, 2026 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	$ 8,489.7
Unrealized Losses within 12 months, Unrealized Losses	(40.8)
U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	2,113.7
Unrealized Losses within 12 months, Unrealized Losses	(9.4)
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	5,108.9
Unrealized Losses within 12 months, Unrealized Losses	(23.0)
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	17.4
Unrealized Losses within 12 months, Unrealized Losses	(0.1)
Agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	1,249.7
Unrealized Losses within 12 months, Unrealized Losses	$ (8.3)