Investment Securities - Debt Securities Issued by Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 1,804.7	$ 1,537.7
AAA
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	6.6	7.8
AA
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	103.6	89.9
A
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	835.5	711.7
BBB
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	844.4	716.9
BB
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	6.5	3.2
Other
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 8.1	$ 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