Investments - Contractual Maturities of Bonds (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Cost or Amortized Cost
Due in one year or less	$ 242.3
Due after one year through five years	431.2
Due after five years through ten years	0.0
Due after ten years	0.0
Cost or Amortized Cost	673.5	$ 696.8
Fair Value
Due in one year or less	238.0
Due after one year through five years	412.3
Due after five years through ten years	0.0
Due after ten years	0.0
Fair Value	$ 650.3	$ 691.4