Investments - Amortized Cost and Fair Values (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 645.4	$ 673.5
Gross unrealized gains	0.0	0.1
Gross unrealized losses	(12.0)	(23.3)
Fair Value	633.4	650.3
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	460.2	549.7
Gross unrealized gains	0.0	0.1
Gross unrealized losses	(9.2)	(19.6)
Fair Value	451.0	530.2
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	184.8	121.0
Gross unrealized gains	0.0	0.0
Gross unrealized losses	(2.8)	(3.7)
Fair Value	182.0	117.3
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	0.4	2.8
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Fair Value	$ 0.4	$ 2.8