Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Net realized investment gains, pre-tax	$ 24.1	$ 270.0	$ 30.0
Net unrealized investment gains (losses), pre-tax	109.2	(297.4)	230.5
Net realized and unrealized investment gains (losses), pre-tax	133.3	(27.4)	260.5
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses)	(12.9)	2.7	(47.7)
Net realized and unrealized investment gains (losses), after-tax	$ 120.4	$ (24.7)	$ 212.8