Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Fixed Maturities, Excluding Asset-Backed Securities And Collateralized Loan Obligations
Cost or Amortized Cost
Due in one year or less	$ 319.9
Due after one year through five years	845.0
Due after five years through ten years	303.4
Due after ten years	101.7
Carrying Value
Due in one year or less	315.0
Due after one year through five years	813.9
Due after five years through ten years	287.0
Due after ten years	94.9
Mortgage and asset-backed securities and collateralized loan obligations
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	628.4
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	598.5
Fixed maturity investments
Cost or Amortized Cost
Total fixed maturity investments	2,198.4	$ 2,076.0
Carrying Value
Total fixed maturity investments	$ 2,109.3	$ 1,920.9