Investment Securities - Debt Securities Issued By Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
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