Investment Securities (Net Realized Investment Gain (losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Net realized investment gains (losses), pre-tax	$ (23.9)	$ 35.3	$ 269.7
Net realized investment gains (losses), foreign, pre-tax	11.0	(11.2)	0.3
Net realized investment gains (losses), pre-tax, total	(12.9)	24.1	270.0
Income tax expense attributable to net realized investment gains (losses)	9.1	(8.9)	(45.6)
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	9.1	(8.9)	(45.6)
Net realized investment gains (losses), after-tax	(14.8)	26.4	224.1
Net realized investment gains (losses), foreign, after-tax	11.0	(11.2)	0.3
Net realized investment gains (losses), total, after-tax	(3.8)	15.2	224.4
Fixed maturity investments
Investment securities
Net realized investment gains (losses), pre-tax	(29.8)	(1.6)	(1.9)
Net realized investment gains (losses), foreign, pre-tax	18.2	4.1	0.3
Net realized investment gains (losses), pre-tax, total	(11.6)	2.5	(1.6)
Short-term investments
Investment securities
Net realized investment gains (losses), pre-tax	(0.8)	(0.3)	0.4
Net realized investment gains (losses), foreign, pre-tax	0.0	0.0	0.0
Net realized investment gains (losses), pre-tax, total	(0.8)	(0.3)	0.4
Common equity securities
Investment securities
Net realized investment gains (losses), pre-tax	6.6	18.1	268.5
Net realized investment gains (losses), foreign, pre-tax	0.0	6.0	0.0
Net realized investment gains (losses), pre-tax, total	6.6	24.1	268.5
Other long-term investments
Investment securities
Net realized investment gains (losses), pre-tax	0.1	19.1	2.7
Net realized investment gains (losses), foreign, pre-tax	(7.2)	(21.3)	0.0
Net realized investment gains (losses), pre-tax, total	$ (7.1)	$ (2.2)	$ 2.7