Fair Value Measurements - Unrealized Loss Position (Details) $ in Millions	Sep. 30, 2026 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	$ 10,995.7
Unrealized Losses within 12 months, Unrealized Losses	(116.4)
U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	2,900.8
Unrealized Losses within 12 months, Unrealized Losses	(26.0)
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	6,806.2
Unrealized Losses within 12 months, Unrealized Losses	(73.4)
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	29.3
Unrealized Losses within 12 months, Unrealized Losses	(0.4)
Agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	1,259.4
Unrealized Losses within 12 months, Unrealized Losses	$ (16.6)