Investments - Amortized Cost and Fair Values (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 693.1	$ 673.5
Gross unrealized gains	0.1	0.1
Gross unrealized losses	(16.4)	(23.3)
Fair Value	676.8	650.3
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	537.3	549.7
Gross unrealized gains	0.1	0.1
Gross unrealized losses	(13.1)	(19.6)
Fair Value	524.3	530.2
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	154.3	121.0
Gross unrealized gains	0.0	0.0
Gross unrealized losses	(3.3)	(3.7)
Fair Value	151.0	117.3
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	1.5	2.8
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Fair Value	$ 1.5	$ 2.8