Fair Value Measurements - Unrealized Loss Position (Details) $ in Millions	Mar. 31, 2025 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	$ 2,239.4
Unrealized Losses within 12 months, Unrealized Losses	(5.2)
Unrealized Losses 12 months or greater, Fair Value	2.8
Unrealized Losses 12 months or greater, Unrealized Losses	0.0
Total, Fair Value	2,242.2
Total, Unrealized Losses	(5.2)
U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	859.3
Unrealized Losses within 12 months, Unrealized Losses	(1.6)
Unrealized Losses 12 months or greater, Fair Value	0.0
Unrealized Losses 12 months or greater, Unrealized Losses	0.0
Total, Fair Value	859.3
Total, Unrealized Losses	(1.6)
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	896.6
Unrealized Losses within 12 months, Unrealized Losses	(2.7)
Unrealized Losses 12 months or greater, Fair Value	2.8
Unrealized Losses 12 months or greater, Unrealized Losses	0.0
Total, Fair Value	899.4
Total, Unrealized Losses	(2.7)
Agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Losses within 12 months, Fair Value	483.5
Unrealized Losses within 12 months, Unrealized Losses	(0.9)
Unrealized Losses 12 months or greater, Fair Value	0.0
Unrealized Losses 12 months or greater, Unrealized Losses	0.0
Total, Fair Value	483.5
Total, Unrealized Losses	$ (0.9)