Investment Securities - Debt Securities Issued By Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Investment securities
Fair value investments	$ 992.9	$ 547.4
AAA
Investment securities
Fair value investments	12.0	10.6
AA
Investment securities
Fair value investments	85.0	57.9
A
Investment securities
Fair value investments	490.4	318.3
BBB
Investment securities
Fair value investments	396.8	159.6
BB
Investment securities
Fair value investments	0.0	1.0
Other
Investment securities
Fair value investments	$ 8.7	$ 0.0