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Investment Securities - Investment Holdings (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022
U.S. Government and agency obligations |  |
Investment securities |  |
Cost or Amortized Cost | $ 209.0 | $ 216.6
Gross Unrealized Gains | 0.3 | 0.0
Gross Unrealized Losses | (4.4) | (10.2)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 204.9 | 206.4
Debt securities issued by corporations |  |
Investment securities |  |
Cost or Amortized Cost | 1,085.9 | 1,098.3
Gross Unrealized Gains | 5.6 | 0.6
Gross Unrealized Losses | (45.4) | (78.3)
Net Foreign Currency Gains (Losses) | (1.1) | (1.8)
Carrying Value | 1,045.0 | 1,018.8
Municipal obligations |  |
Investment securities |  |
Cost or Amortized Cost | 275.1 | 281.6
Gross Unrealized Gains | 0.9 | 0.4
Gross Unrealized Losses | (15.1) | (23.4)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 260.9 | 258.6
Mortgage and asset-backed securities |  |
Investment securities |  |
Cost or Amortized Cost | 417.2 | 288.7
Gross Unrealized Gains | 2.0 | 0.0
Gross Unrealized Losses | (29.8) | (34.5)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 389.4 | 254.2
Collateralized loan obligations |  |
Investment securities |  |
Cost or Amortized Cost | 211.2 | 190.8
Gross Unrealized Gains | 0.4 | 0.1
Gross Unrealized Losses | (2.4) | (6.0)
Net Foreign Currency Gains (Losses) | (0.1) | (2.0)
Carrying Value | 209.1 | 182.9
Fixed maturity investments |  |
Investment securities |  |
Cost or Amortized Cost | 2,198.4 | 2,076.0
Gross Unrealized Gains | 9.2 | 1.1
Gross Unrealized Losses | (97.1) | (152.4)
Net Foreign Currency Gains (Losses) | (1.2) | (3.8)
Carrying Value | $ 2,109.3 | $ 1,920.9

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