Investment Securities (Debt Securities Issued By Corporation) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Fair value investments	$ 5,039.7	$ 4,050.9	$ 3,343.6
Debt securities issued by corporations
Investment securities
Fair value investments	1,509.4	1,000.0
BBB | Debt securities issued by corporations
Investment securities
Fair value investments	786.5	507.1
A | Debt securities issued by corporations
Investment securities
Fair value investments	381.9	397.7
AA | Debt securities issued by corporations
Investment securities
Fair value investments	100.9	95.2
Standard & Poor's, BB Rating [Member] | Debt securities issued by corporations
Investment securities
Fair value investments	214.0	0.0
Standard & Poor's, B Rating [Member] | Debt securities issued by corporations
Investment securities
Fair value investments	$ 26.1	$ 0.0