Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) $ in Millions	Dec. 31, 2020 USD ($)
Cost or Amortized Cost
Total	$ 1,151.7
Carrying Value
Total	1,207.2
Fixed Maturities and Convertible Fixed Maturities Excluding Mortgage Backed Securities
Cost or Amortized Cost
Due in one year or less	126.0
Due after one year through five years	417.5
Due after five years through ten years	283.2
Due after ten years	113.3
Carrying Value
Due in one year or less	127.0
Due after one year through five years	433.6
Due after five years through ten years	303.2
Due after ten years	124.9
Mortgage and asset-backed securities
Cost or Amortized Cost
Mortgage and asset-backed securities	211.7
Carrying Value
Mortgage and asset-backed securities	$ 218.5