Investments - Contractual Maturities of Bonds (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Cost or Amortized Cost
Due in one year or less	$ 216.7
Due after one year through five years	462.4
Due after five years through ten years	0.2
Due after ten years	0.0
Cost or Amortized Cost	679.3	$ 607.1
Fair Value
Due in one year or less	217.4
Due after one year through five years	464.2
Due after five years through ten years	0.2
Due after ten years	0.0
Fair Value	$ 681.8	$ 607.4