Investments - Contractual Maturities of Bonds (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Cost or Amortized Cost
Due in one year or less	$ 309.7
Due after one year through five years	344.2
Due after five years through ten years	0.2
Due after ten years	0.0
Cost or Amortized Cost	654.1	$ 607.1
Fair Value
Due in one year or less	310.1
Due after one year through five years	345.3
Due after five years through ten years	0.2
Due after ten years	0.0
Fair Value	$ 655.6	$ 607.4