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Investment Securities - Debt Securities Issued by Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
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Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | $ 1,073.1 | $ 1,045.0
AAA |  |
Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | 11.4 | 11.5
AA |  |
Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | 83.5 | 83.8
A |  |
Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | 562.4 | 552.4
BBB |  |
Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | 407.5 | 390.9
BB |  |
Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | 1.8 | 0.0
Other |  |
Debt Securities, Trading, Credit Quality Indicator [Line Items] |  |
Fixed maturity investments, at fair value | $ 6.5 | $ 6.4

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