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Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) $ in Millions	Sep. 30, 2021 USD ($)
Cost or Amortized Cost
Total fixed maturity investments	$ 1,804.5
Carrying Value
Total fixed maturity investments	1,832.9
Fixed Maturities and Convertible Fixed Maturities Excluding Mortgage Backed Securities
Cost or Amortized Cost
Due in one year or less	111.9
Due after one year through five years	836.1
Due after five years through ten years	368.5
Due after ten years	112.2
Carrying Value
Due in one year or less	112.7
Due after one year through five years	843.4
Due after five years through ten years	377.3
Due after ten years	121.8
Mortgage and asset-backed securities
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	375.8
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	$ 377.7

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