Investments - Contractual Maturities of Bonds (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Cost or Amortized Cost
Due in one year or less	$ 258.7
Due after one year through five years	373.3
Due after five years through ten years	0.0
Due after ten years	0.0
Cost or Amortized Cost	632.0	$ 673.5
Fair Value
Due in one year or less	255.5
Due after one year through five years	371.9
Due after five years through ten years	0.0
Due after ten years	0.0
Fair Value	$ 627.4	$ 650.3