Investments - Amortized Cost and Fair Values (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 540.5	$ 632.0
Gross Unrealized Gains	0.3	1.7
Gross Unrealized Losses	(3.6)	(6.3)
Fair Value	537.2	627.4
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	438.8	453.6
Gross Unrealized Gains	0.3	1.3
Gross Unrealized Losses	(3.2)	(5.0)
Fair Value	435.9	449.9
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	88.9	176.8
Gross Unrealized Gains	0.0	0.4
Gross Unrealized Losses	(0.4)	(1.3)
Fair Value	88.5	175.9
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	12.8	1.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 12.8	$ 1.6