Investments - Amortized Cost and Fair Values (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 654.1	$ 607.1
Gross Unrealized Gains	2.1	1.6
Gross Unrealized Losses	(0.6)	(1.3)
Fair Value	655.6	607.4
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	519.3	470.6
Gross Unrealized Gains	1.7	1.3
Gross Unrealized Losses	(0.5)	(1.1)
Fair Value	520.5	470.8
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	100.3	107.6
Gross Unrealized Gains	0.4	0.3
Gross Unrealized Losses	(0.1)	(0.2)
Fair Value	100.6	107.7
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	27.0	22.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	27.0	22.9
Non-U.S. government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	7.5	6.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 7.5	$ 6.0