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Investment Securities - Investment Holdings Fixed Maturity Table (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2023 | Mar. 31, 2022
U.S. Government and agency obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | $ 245.7 | $ 216.6
Gross Unrealized Gains | 0.1 | 0.0
Gross Unrealized Losses | (8.0) | (10.2)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 237.8 | 206.4
Debt securities issued by corporations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 1,073.9 | 1,098.3
Gross Unrealized Gains | 1.3 | 0.6
Gross Unrealized Losses | (63.1) | (78.3)
Net Foreign Currency Gains (Losses) | (1.7) | (1.8)
Carrying Value | 1,010.4 | 1,018.8
Municipal obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 280.0 | 281.6
Gross Unrealized Gains | 0.0 | 0.4
Gross Unrealized Losses | (17.0) | (23.4)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 263.0 | 258.6
Mortgage and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 289.5 | 288.7
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (30.7) | (34.5)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 258.8 | 254.2
Collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 192.1 | 190.8
Gross Unrealized Gains | 0.1 | 0.1
Gross Unrealized Losses | (5.2) | (6.0)
Net Foreign Currency Gains (Losses) | (1.1) | (2.0)
Carrying Value | 185.9 | 182.9
Total fixed maturity investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 2,081.2 | 2,076.0
Gross Unrealized Gains | 1.5 | 1.1
Gross Unrealized Losses | (124.0) | (152.4)
Net Foreign Currency Gains (Losses) | (2.8) | (3.8)
Carrying Value | $ 1,955.9 | $ 1,920.9

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