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Investment Securities - Debt Securities Issued by Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions | Mar. 31, 2025 | Dec. 31, 2024
--- | --- | ---
Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | $ 1,398.5 | $ 1,414.2
AAA |  |
Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | 11.9 | 14.6
AA |  |
Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | 89.5 | 91.2
A |  |
Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | 623.7 | 607.2
BBB |  |
Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | 659.1 | 688.7
BB |  |
Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | 6.8 | 5.4
Other |  |
Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | $ 7.5 | $ 7.1

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