Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Fixed Maturities, Excluding Asset-Backed Securities And Collateralized Loan Obligations
Cost or Amortized Cost
Due in one year or less	$ 205.8	$ 319.9
Due after one year through five years	1,494.5	845.0
Due after five years through ten years	206.1	303.4
Due after ten years	25.3	101.7
Carrying Value
Due in one year or less	203.9	315.0
Due after one year through five years	1,478.7	813.9
Due after five years through ten years	193.2	287.0
Due after ten years	25.2	94.9
Mortgage and asset-backed securities and collateralized loan obligations
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	637.5	628.4
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	610.6	598.5
Fixed maturity investments
Cost or Amortized Cost
Total fixed maturity investments	2,569.2	2,198.4
Carrying Value
Total fixed maturity investments	$ 2,511.6	$ 2,109.3