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Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
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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

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