Investments - Amortized Cost and Fair Values (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 632.0	$ 673.5
Gross unrealized gains	1.7	0.1
Gross unrealized losses	(6.3)	(23.3)
Fair Value	627.4	650.3
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	453.6	549.7
Gross unrealized gains	1.3	0.1
Gross unrealized losses	(5.0)	(19.6)
Fair Value	449.9	530.2
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	176.8	121.0
Gross unrealized gains	0.4	0.0
Gross unrealized losses	(1.3)	(3.7)
Fair Value	175.9	117.3
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	1.6	2.8
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Fair Value	$ 1.6	$ 2.8