Investments - Contractual Maturities of Bonds (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Cost or Amortized Cost
Due in one year or less	$ 307.0
Due after one year through five years	386.1
Due after five years through ten years	0.0
Due after ten years	0.0
Cost or Amortized Cost	693.1	$ 673.5
Fair Value
Due in one year or less	300.4
Due after one year through five years	376.4
Due after five years through ten years	0.0
Due after ten years	0.0
Fair Value	$ 676.8	$ 650.3