Investments - Contractual Maturities of Bonds (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Cost or Amortized Cost
Due in one year or less	$ 257.8
Due after one year through five years	387.6
Due after five years through ten years	0.0
Due after ten years	0.0
Cost or Amortized Cost	645.4	$ 673.5
Fair Value
Due in one year or less	252.9
Due after one year through five years	380.5
Due after five years through ten years	0.0
Due after ten years	0.0
Fair Value	$ 633.4	$ 650.3