Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Fixed Maturities, Excluding Asset-Backed Securities And Collateralized Loan Obligations
Cost or Amortized Cost
Due in one year or less	$ 617.4	$ 205.8
Due after one year through five years	1,120.8	1,494.5
Due after five years through ten years	199.5	206.1
Due after ten years	16.4	25.3
Carrying Value
Due in one year or less	617.1	203.9
Due after one year through five years	1,126.8	1,478.7
Due after five years through ten years	198.5	193.2
Due after ten years	16.6	25.2
Mortgage and asset-backed securities and collateralized loan obligations
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	769.5	637.5
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	759.4	610.6
Fixed maturity investments
Cost or Amortized Cost
Total fixed maturity investments	2,723.6	2,569.2
Carrying Value
Total fixed maturity investments	$ 2,718.4	$ 2,511.6