Debt Securities - Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	$ 678.2	$ 265.1
Less Than 12 Months, Unrealized Loss	(2.3)	(0.4)
12 Months or Longer, Fair Value	104.0	183.8
12 Months or Longer, Unrealized Loss	(0.6)	(1.0)
Total, Fair Value	782.2	448.9
Total, Unrealized Loss	(2.9)	(1.4)
Securities of government-sponsored entities
Debt Securities, Available-for-sale [Line Items]
Less Than 12 Months, Fair Value	386.7	214.6
Less Than 12 Months, Unrealized Loss	(1.0)	(0.2)
12 Months or Longer, Fair Value	51.0	16.7
12 Months or Longer, Unrealized Loss	(0.2)	(0.4)
Total, Fair Value	437.7	231.3
Total, Unrealized Loss	$ (1.2)	$ (0.6)