Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Net realized investment gains, pre-tax	$ 77.3	$ 166.8	$ 95.5
Net unrealized investment gains (losses), pre-tax	148.1	(88.3)	38.4
Net realized and unrealized investment gains, pre-tax	225.4	78.5	133.9
Income tax expense attributable to net realized and unrealized investment gains	(35.2)	(18.0)	(12.8)
Net realized and unrealized investment gains, after tax	$ 190.2	$ 60.5	$ 121.1