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Investment Securities - Investment Holdings Fixed Maturity Table (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Debt and Equity Securities, FV-NI [Line Items] |  |
Net Foreign Currency Gains | $ 0.1 |
U.S. Government and agency obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 220.1 | $ 173.2
Gross Unrealized Gains | 2.1 | 3.1
Gross Unrealized Losses | (0.6) | 0.0
Net Foreign Currency Gains | 0.0 |
Carrying Value | 221.6 | 176.3
Debt securities issued by corporations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 678.6 | 522.8
Gross Unrealized Gains | 13.9 | 24.7
Gross Unrealized Losses | (5.7) | (0.1)
Net Foreign Currency Gains | 0.1 |
Carrying Value | 686.9 | 547.4
Municipal obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 269.4 | 244.0
Gross Unrealized Gains | 15.4 | 21.0
Gross Unrealized Losses | (1.0) | 0.0
Net Foreign Currency Gains | 0.0 |
Carrying Value | 283.8 | 265.0
Mortgage and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 225.8 | 211.7
Gross Unrealized Gains | 5.4 | 6.8
Gross Unrealized Losses | (1.3) | 0.0
Net Foreign Currency Gains | 0.0 |
Carrying Value | 229.9 | 218.5
Total fixed maturity investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 1,453.4 | 1,151.7
Gross Unrealized Gains | 36.8 | 55.6
Gross Unrealized Losses | (8.7) | (0.1)
Carrying Value | 1,481.6 | $ 1,207.2
Collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or Amortized Cost | 59.5 |
Gross Unrealized Gains | 0.0 |
Gross Unrealized Losses | (0.1) |
Net Foreign Currency Gains | 0.0 |
Carrying Value | $ 59.4 |

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