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Investment Securities - Debt Securities Issued By Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions | Dec. 31, 2025 | Dec. 31, 2024
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Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | $ 1,537.7 | $ 1,414.2
AAA |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 7.8 | 14.6
AA |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 89.9 | 91.2
A |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 711.7 | 607.2
BBB |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 716.9 | 688.7
BB |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | 3.2 | 5.4
Other |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed maturity investments, at fair value | $ 8.2 | $ 7.1

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