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Investment Securities - Mortgage and Asset-Backed Securities and Collateralized Loan Obligations (Details) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Total mortgage and asset-backed securities and collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | $ 696.6 | $ 598.5
Total mortgage and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 450.1 | 389.4
Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 404.5 | 366.4
Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 45.6 | 23.0
Collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 246.5 | 209.1
Level 2 | Total mortgage and asset-backed securities and collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 696.6 | 598.5
Level 2 | Total mortgage and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 450.1 | 389.4
Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 404.5 | 366.4
Level 2 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 45.6 | 23.0
Level 2 | Collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 246.5 | 209.1
Level 3 | Total mortgage and asset-backed securities and collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Level 3 | Total mortgage and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Level 3 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Level 3 | Collateralized loan obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Total non-agency | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.1 | 0.2
Total non-agency | Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.1 | 0.2
Total non-agency | Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Total agency | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 404.4 | 366.2
Total agency | Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 404.4 | 366.2
Total agency | Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
FNMA | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 216.7 | 198.8
FNMA | Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 216.7 | 198.8
FNMA | Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
FHLMC | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 147.8 | 137.2
FHLMC | Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 147.8 | 137.2
FHLMC | Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
GNMA | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 39.9 | 30.2
GNMA | Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 39.9 | 30.2
GNMA | Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Non-agency: Residential | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.1 | 0.2
Non-agency: Residential | Level 2 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.1 | 0.2
Non-agency: Residential | Level 3 | Mortgage-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Vehicle receivables | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 31.1 | 17.2
Vehicle receivables | Level 2 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 31.1 | 17.2
Vehicle receivables | Level 3 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Credit card receivables | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 3.3 | 3.3
Credit card receivables | Level 2 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 3.3 | 3.3
Credit card receivables | Level 3 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Other | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 11.2 | 2.5
Other | Level 2 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 11.2 | 2.5
Other | Level 3 | Other asset-backed securities: |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | $ 0.0 | $ 0.0

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