Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) - USD ($) $ in Millions	Mar. 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	$ 300.5
Due after one year through five years	886.3
Due after five years through ten years	295.7
Due after ten years	134.8
Carrying Value
Due in one year or less	296.1
Due after one year through five years	856.8
Due after five years through ten years	277.8
Due after ten years	127.3
Mortgage and asset-backed securities and collateralized loan obligations
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	733.7
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	696.6
Fixed maturity investments
Cost or Amortized Cost
Total fixed maturity investments	2,351.0	$ 2,198.4
Carrying Value
Total fixed maturity investments	$ 2,254.6	$ 2,109.3