Investments - Amortized Cost and Fair Values (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 607.1
Gross unrealized gains	1.6	$ 1.7
Gross unrealized losses	(1.3)	(6.3)
Fair Value	607.4	627.4
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	470.6	453.6
Gross unrealized gains	1.3	1.3
Gross unrealized losses	(1.1)	(5.0)
Fair Value	470.8	449.9
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	107.6	176.8
Gross unrealized gains	0.3	0.4
Gross unrealized losses	(0.2)	(1.3)
Fair Value	107.7	175.9
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	22.9	1.6
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Fair Value	22.9	1.6
Non-U.S. government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	6.0	0.0
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Fair Value	$ 6.0	$ 0.0