Investment Securities (Debt Securities Issued By Corporation) (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
Debt securities issued by corporations
Investment securities
Fair value investments	880.9	746.3
AAA | Debt securities issued by corporations
Investment securities
Fair value investments	1.6	0.0
AA | Debt securities issued by corporations
Investment securities
Fair value investments	42.6	37.3
A | Debt securities issued by corporations
Investment securities
Fair value investments	192.5	212.8
BBB | Debt securities issued by corporations
Investment securities
Fair value investments	465.2	335.6
BB | Debt securities issued by corporations
Investment securities
Fair value investments	161.7	143.2
B | Debt securities issued by corporations
Investment securities
Fair value investments	$ 17.3	17.4
Star & Shield Insurance Exchange | Fixed maturity investments
Investment securities
Fair value investments		6.6
Discontinued Operations, Held-for-sale | Star & Shield Insurance Exchange | Fixed maturity investments
Investment securities
Fair value investments		$ 4.2