Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) $ in Millions	Mar. 31, 2023 USD ($)
Cost or Amortized Cost
Total fixed maturity investments	$ 2,081.2
Carrying Value
Total fixed maturity investments	1,955.9
Fixed Maturities and Convertible Fixed Maturities Excluding Mortgage Backed Securities
Cost or Amortized Cost
Due in one year or less	223.8
Due after one year through five years	943.6
Due after five years through ten years	338.8
Due after ten years	93.4
Carrying Value
Due in one year or less	219.2
Due after one year through five years	891.9
Due after five years through ten years	314.2
Due after ten years	85.9
Mortgage and asset-backed securities
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	481.6
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	$ 444.7