Investment Securities (Other Long-Term Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Cost	$ 5,308.8
Other Investments	323.3	$ 315.8
Other long-term investments
Investment securities
Other Investments	2.0	3.2
Limited liability companies and private equity securities [Member]
Investment securities
Other Investments	72.0	82.1
Surplus Note
Investment securities
Other Investments	71.9	51.5
Convertible Preferred Stock
Investment securities
Other Investments	30.6	32.7
Affordable Housing Development Fund [Member]
Investment securities
Other Investments	12.3	14.7
Partnership investments [Member]
Investment securities
Other Investments	3.5	3.8
Total hedge and private equity funds included in other long-term investments
Investment securities
Other Investments	$ 131.0	$ 127.8