Investment Securities (Debt Securities Issued By Corporation) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investment securities
Fair value investments	$ 2,746.0	$ 2,328.7	$ 3,208.3
Debt securities issued by corporations
Investment securities
Fair value investments	467.2	510.5
AAA | Debt securities issued by corporations
Investment securities
Fair value investments	9.5	8.9
AA | Debt securities issued by corporations
Investment securities
Fair value investments	73.9	88.7
A | Debt securities issued by corporations
Investment securities
Fair value investments	288.5	270.5
BBB | Debt securities issued by corporations
Investment securities
Fair value investments	$ 95.3	$ 142.4