Investment Securities - Debt Securities Issued by Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 1,781.5	$ 1,537.7
AAA
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	3.3	7.8
AA
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	98.9	89.9
A
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	855.7	711.7
BBB
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	812.2	716.9
BB
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	3.4	3.2
Other
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 8.0	$ 8.2

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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+ Details