Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) - USD ($) $ in Millions	Jun. 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	$ 229.8
Due after one year through five years	971.1
Due after five years through ten years	306.1
Due after ten years	141.8
Carrying Value
Due in one year or less	226.6
Due after one year through five years	943.9
Due after five years through ten years	288.0
Due after ten years	132.7
Mortgage and asset-backed securities and collateralized loan obligations
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	792.2
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	753.6
Fixed maturity investments
Cost or Amortized Cost
Total fixed maturity investments	2,441.0	$ 2,198.4
Carrying Value
Total fixed maturity investments	$ 2,344.8	$ 2,109.3