Investment Securities (Summary of Non-agency Commercial Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Fair value investments	$ 3,208.3	$ 2,547.3	$ 1,543.5
Commercial
Investment securities
Fair value investments	70.5	3.9
Total Non-agency
Investment securities
Fair value investments	70.5	$ 74.2
2017 | Commercial
Investment securities
Fair value investments	67.0
2016 | Commercial
Investment securities
Fair value investments	$ 3.5