Investment Securities - Debt Securities Issued By Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Investment securities
Fair value investments	$ 1,018.8	$ 992.9
AAA
Investment securities
Fair value investments	11.3	12.0
AA
Investment securities
Fair value investments	96.0	85.0
A
Investment securities
Fair value investments	567.9	490.4
BBB
Investment securities
Fair value investments	337.7	396.8
Other
Investment securities
Fair value investments	$ 5.9	$ 8.7