Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Net realized investment gains, pre-tax	$ 6.8	$ 0.7	$ 20.8	$ 265.0
Net unrealized investment gains (losses), pre-tax	25.7	10.2	81.7	(237.8)
Net realized and unrealized investment gains, pre-tax	32.5	10.9	102.5	27.2
Income tax expense attributable to net realized and unrealized investment gains	(3.9)	0.0	(9.5)	(4.0)
Net realized and unrealized investment gains, after tax	$ 28.6	$ 10.9	$ 93.0	$ 23.2