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Investment Securities - Investment Holdings Fixed Maturity Table (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2023 | Dec. 31, 2022
U.S. Government and agency obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | $ 227.6 | $ 216.6
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (9.4) | (10.2)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 218.2 | 206.4
Debt securities issued by corporations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 1,061.8 | 1,098.3
Gross Unrealized Gains | 0.3 | 0.6
Gross Unrealized Losses | (67.9) | (78.3)
Net Foreign Currency Gains (Losses) | (1.7) | (1.8)
Carrying Value | 992.5 | 1,018.8
Municipal obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 273.6 | 281.6
Gross Unrealized Gains | 0.0 | 0.4
Gross Unrealized Losses | (19.4) | (23.4)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 254.2 | 258.6
Mortgage and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 321.9 | 288.7
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (33.9) | (34.5)
Net Foreign Currency Gains (Losses) | 0.0 | 0.0
Carrying Value | 288.0 | 254.2
Collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 192.4 | 190.8
Gross Unrealized Gains | 0.2 | 0.1
Gross Unrealized Losses | (3.6) | (6.0)
Net Foreign Currency Gains (Losses) | (1.2) | (2.0)
Carrying Value | 187.8 | 182.9
Total fixed maturity investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 2,077.3 | 2,076.0
Gross Unrealized Gains | 0.5 | 1.1
Gross Unrealized Losses | (134.2) | (152.4)
Net Foreign Currency Gains (Losses) | (2.9) | (3.8)
Carrying Value | $ 1,940.7 | $ 1,920.9

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