Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Net realized investment (losses) gains, pre-tax	$ (5.1)	$ 0.6
Net unrealized investment (losses) gains, pre-tax	(48.6)	35.7
Net realized and unrealized investment (losses) gains, pre-tax	(53.7)	36.3
Income tax benefit (expense) attributable to net realized and unrealized investment (losses) gains	5.7	(3.9)
Net realized and unrealized investment (losses) gains, after-tax	$ (48.0)	$ 32.4