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Investment Securities - Fixed Maturity Investments (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2024 | Dec. 31, 2023
Fixed maturity investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | $ 2,434.1 | $ 2,198.4
Gross Unrealized Gains | 27.4 | 9.2
Gross Unrealized Losses | (41.5) | (97.1)
Net Foreign Currency Gains (Losses) | 2.3 | (1.2)
Carrying Value | 2,422.3 | 2,109.3
U.S. Government and agency obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 446.0 | 209.0
Gross Unrealized Gains | 4.8 | 0.3
Gross Unrealized Losses | (0.6) | (4.4)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 450.2 | 204.9
Debt securities issued by corporations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 1,318.3 | 1,085.9
Gross Unrealized Gains | 16.1 | 5.6
Gross Unrealized Losses | (24.4) | (45.4)
Net Foreign Currency Gains (Losses) | 0.9 | (1.1)
Carrying Value | 1,310.9 | 1,045.0
Municipal obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 3.2 | 275.1
Gross Unrealized Gains | 0.1 | 0.9
Gross Unrealized Losses | 0.0 | (15.1)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 3.3 | 260.9
Mortgage and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 379.0 | 417.2
Gross Unrealized Gains | 5.2 | 2.0
Gross Unrealized Losses | (16.1) | (29.8)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 368.1 | 389.4
Collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 287.6 | 211.2
Gross Unrealized Gains | 1.2 | 0.4
Gross Unrealized Losses | (0.4) | (2.4)
Net Foreign Currency Gains (Losses) | 1.4 | (0.1)
Carrying Value | $ 289.8 | $ 209.1

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