Investment Securities (Debt Securities Issued By Corporation) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Fair value investments	$ 4,050.9	$ 3,343.6	$ 3,889.0
Debt securities issued by corporations
Investment securities
Fair value investments	1,000.0	1,171.7
Other Standard Poors Rating [Member] | Debt securities issued by corporations
Investment securities
Fair value investments	0.0	5.4
BBB | Debt securities issued by corporations
Investment securities
Fair value investments	507.1	426.5
A | Debt securities issued by corporations
Investment securities
Fair value investments	397.7	594.9
AA | Debt securities issued by corporations
Investment securities
Fair value investments	$ 95.2	$ 144.9