Investments - Contractual Maturities of Bonds (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Cost or Amortized Cost
Due in one year or less	$ 224.8
Due after one year through five years	382.2
Due after five years through ten years	0.0
Due after ten years	0.0
Cost or Amortized Cost	607.0	$ 632.0
Fair Value
Due in one year or less	224.4
Due after one year through five years	385.3
Due after five years through ten years	0.0
Due after ten years	0.0
Fair Value	$ 609.7	$ 627.4