Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) $ in Millions	Mar. 31, 2022 USD ($)
Cost or Amortized Cost
Total fixed maturity investments	$ 1,881.7
Carrying Value
Total fixed maturity investments	1,815.5
Fixed Maturities and Convertible Fixed Maturities Excluding Mortgage Backed Securities
Cost or Amortized Cost
Due in one year or less	157.7
Due after one year through five years	858.2
Due after five years through ten years	338.4
Due after ten years	112.7
Carrying Value
Due in one year or less	157.1
Due after one year through five years	825.6
Due after five years through ten years	321.2
Due after ten years	114.1
Mortgage and asset-backed securities
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	414.7
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	$ 397.5