Investment Securities (Debt Securities Issued By Corporation) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Fair value investments	$ 2,328.7	$ 3,208.3	$ 2,547.3
Debt securities issued by corporations
Investment securities
Fair value investments	510.5	880.9
AAA | Debt securities issued by corporations
Investment securities
Fair value investments	8.9	1.6
AA | Debt securities issued by corporations
Investment securities
Fair value investments	88.7	42.6
A | Debt securities issued by corporations
Investment securities
Fair value investments	270.5	192.5
BBB | Debt securities issued by corporations
Investment securities
Fair value investments	142.4	465.2
BB | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	161.7
B | Debt securities issued by corporations
Investment securities
Fair value investments	$ 0.0	$ 17.3