Investment Securities - Debt Securities Issued By Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Investment securities
Fixed maturity investments, at fair value	$ 1,045.0	$ 1,018.8
AAA
Investment securities
Fixed maturity investments, at fair value	11.5	11.3
AA
Investment securities
Fixed maturity investments, at fair value	83.8	96.0
A
Investment securities
Fixed maturity investments, at fair value	552.4	567.9
BBB
Investment securities
Fixed maturity investments, at fair value	390.9	337.7
Other
Investment securities
Fixed maturity investments, at fair value	$ 6.4	$ 5.9