Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Net realized investment gains, pre-tax	$ 13.4	$ 1.6	$ 14.0	$ 264.3
Net unrealized investment gains (losses), pre-tax	20.3	1.8	56.0	(248.0)
Net realized and unrealized investment gains, pre-tax	33.7	3.4	70.0	16.3
Income tax expense attributable to net realized and unrealized investment gains	(1.7)	(1.4)	(5.5)	(3.9)
Net realized and unrealized investment gains, after tax	$ 32.0	$ 2.0	$ 64.5	$ 12.4