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Investment Securities - Fixed Maturity Investments (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2024 | Dec. 31, 2023
Fixed maturity investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | $ 2,351.0 | $ 2,198.4
Gross Unrealized Gains | 5.0 | 9.2
Gross Unrealized Losses | (97.6) | (97.1)
Net Foreign Currency Gains (Losses) | (3.8) | (1.2)
Carrying Value | 2,254.6 | 2,109.3
U.S. Government and agency obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 247.6 | 209.0
Gross Unrealized Gains | 0.1 | 0.3
Gross Unrealized Losses | (1.8) | (4.4)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 245.9 | 204.9
Debt securities issued by corporations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 1,118.3 | 1,085.9
Gross Unrealized Gains | 3.0 | 5.6
Gross Unrealized Losses | (46.0) | (45.4)
Net Foreign Currency Gains (Losses) | (2.2) | (1.1)
Carrying Value | 1,073.1 | 1,045.0
Municipal obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 251.4 | 275.1
Gross Unrealized Gains | 0.6 | 0.9
Gross Unrealized Losses | (13.0) | (15.1)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 239.0 | 260.9
Mortgage and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 484.8 | 417.2
Gross Unrealized Gains | 0.6 | 2.0
Gross Unrealized Losses | (35.3) | (29.8)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 450.1 | 389.4
Collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 248.9 | 211.2
Gross Unrealized Gains | 0.7 | 0.4
Gross Unrealized Losses | (1.5) | (2.4)
Net Foreign Currency Gains (Losses) | (1.6) | (0.1)
Carrying Value | $ 246.5 | $ 209.1

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