Investment Securities Investment Securities (Net Realized Investment Gain (losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Net realized investment gains (losses), pre-tax	$ 35.3	$ 269.7	$ 30.0
Net realized investment gains (losses), foreign, pre-tax	(11.2)	0.3	0.0
Net realized investment gains, pre-tax, total	24.1	270.0	30.0
Income tax expense attributable to net realized investment gains (losses)	(8.9)	(45.6)	(6.4)
Income tax expense attributable to net realized investment gains (losses), foreign	0.0	0.0	0.0
Income tax expense attributable to net realized investment gains, total	(8.9)	(45.6)	(6.4)
Net realized investment gains (losses), after-tax	26.4	224.1	23.6
Net realized investment gains (losses), foreign, after-tax	(11.2)	0.3	0.0
Net realized investment gains (losses), total, after-tax	15.2	224.4	23.6
Fixed maturity investments
Investment securities
Net realized investment gains (losses), pre-tax	(1.6)	(1.9)	(0.2)
Net realized investment gains (losses), foreign, pre-tax	4.1	0.3	0.0
Net realized investment gains, pre-tax, total	2.5	(1.6)	(0.2)
Short-term investments
Investment securities
Net realized investment gains (losses), pre-tax	(0.3)	0.4
Net realized investment gains (losses), foreign, pre-tax	0.0	0.0
Net realized investment gains, pre-tax, total	(0.3)	0.4
Common equity securities
Investment securities
Net realized investment gains (losses), pre-tax	18.1	268.5	31.0
Net realized investment gains (losses), foreign, pre-tax	6.0	0.0	0.0
Net realized investment gains, pre-tax, total	24.1	268.5	31.0
Other long-term investments
Investment securities
Net realized investment gains (losses), pre-tax	19.1	2.7	(0.8)
Net realized investment gains (losses), foreign, pre-tax	(21.3)	0.0	0.0
Net realized investment gains, pre-tax, total	$ (2.2)	$ 2.7	$ (0.8)