Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Fixed Maturities, Excluding Asset-Backed Securities And Collateralized Loan Obligations
Cost or Amortized Cost
Due in one year or less	$ 237.0
Due after one year through five years	1,277.8
Due after five years through ten years	228.4
Due after ten years	24.2
Carrying Value
Due in one year or less	235.1
Due after one year through five years	1,280.7
Due after five years through ten years	223.3
Due after ten years	25.3
Mortgage and asset-backed securities and collateralized loan obligations
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	666.7
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	657.9
Fixed maturity investments
Cost or Amortized Cost
Total fixed maturity investments	2,434.1	$ 2,198.4
Carrying Value
Total fixed maturity investments	$ 2,422.3	$ 2,109.3