Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Net realized investment gains (losses), pre-tax	$ 1.4	$ 13.4	$ (3.7)	$ 14.0
Net unrealized investment gains (losses), pre-tax	9.1	20.3	(39.5)	56.0
Net realized and unrealized investment gains (losses), pre-tax	10.5	33.7	(43.2)	70.0
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses)	(1.9)	(1.7)	3.8	(5.5)
Net realized and unrealized investment gains (losses), after-tax	$ 8.6	$ 32.0	$ (39.4)	$ 64.5