Investment Securities - Debt Securities Issued By Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Investment securities
Fixed maturity investments, at fair value	$ 1,414.2	$ 1,045.0
AAA
Investment securities
Fixed maturity investments, at fair value	14.6	11.5
AA
Investment securities
Fixed maturity investments, at fair value	91.2	83.8
A
Investment securities
Fixed maturity investments, at fair value	607.2	552.4
BBB
Investment securities
Fixed maturity investments, at fair value	688.7	390.9
BB
Investment securities
Fixed maturity investments, at fair value	5.4	0.0
Other
Investment securities
Fixed maturity investments, at fair value	$ 7.1	$ 6.4