Investments - Amortized Cost and Fair Values (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 607.0	$ 632.0
Gross Unrealized Gains	4.0	1.7
Gross Unrealized Losses	(1.3)	(6.3)
Fair Value	609.7	627.4
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	501.5	453.6
Gross Unrealized Gains	3.3	1.3
Gross Unrealized Losses	(1.3)	(5.0)
Fair Value	503.5	449.9
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	87.2	176.8
Gross Unrealized Gains	0.6	0.4
Gross Unrealized Losses	0.0	(1.3)
Fair Value	87.8	175.9
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	18.3	1.6
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 18.4	$ 1.6