Investments - Contractual Maturities of Bonds (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Cost or Amortized Cost
Due in one year or less	$ 69.2
Due after one year through five years	627.6
Due after five years through ten years	0.0
Due after ten years	0.0
Amortized cost, available-for-sale securities	696.8	$ 6.4
Fair Value
Due in one year or less	69.2
Due after one year through five years	622.2
Due after five years through ten years	0.0
Due after ten years	0.0
Fair Value	$ 691.4	$ 6.6