Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Net realized investment gains, pre-tax	$ 280.3	$ 77.3	$ 166.8
Net unrealized investment (losses) gains, pre-tax	(270.0)	148.1	(88.3)
Net realized and unrealized investment gains, pre-tax	10.3	225.4	78.5
Income tax expense attributable to net realized and unrealized investment gains	(8.8)	(35.2)	(18.0)
Net realized and unrealized investment gains, after tax	$ 1.5	$ 190.2	$ 60.5