Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Net realized investment (losses) gains, pre-tax	$ (12.9)	$ 24.1	$ 270.0
Net unrealized investment (losses) gains, pre-tax	(95.4)	109.2	(297.4)
Net realized and unrealized investment (losses) gains, pre-tax	(108.3)	133.3	(27.4)
Income tax benefit (expense) attributable to net realized and unrealized investment (losses) gains	18.2	(12.9)	2.7
Net realized and unrealized investment (losses) gains, after-tax	$ (90.1)	$ 120.4	$ (24.7)