Investments - Contractual Maturities of Bonds (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Cost or Amortized Cost
Due in one year or less	$ 225.1
Due after one year through five years	398.4
Due after five years through ten years	0.0
Due after ten years	0.0
Cost or Amortized Cost	623.5	$ 632.0
Fair Value
Due in one year or less	223.7
Due after one year through five years	396.1
Due after five years through ten years	0.0
Due after ten years	0.0
Fair Value	$ 619.8	$ 627.4