Accumulated Other Comprehensive Income (Reclassification out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized (losses) gains on investments, excluding other-than-temporary impairment losses	$ (5.2)		$ (16.1)
Net realized (losses) gains on investments, excluding other-than-temporary impairment losses		$ 8.9		$ 26.0
Interest expense	26.5	12.2	74.0	37.2
Total reclassifications for the period, net of tax	4.4	(2.9)	12.1	(13.4)
Net unrealized gains on securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period, net of tax	4.0	(3.5)	11.4	(14.3)
Unamortized net (losses) on Pension Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision for income taxes			(0.5)	(0.5)
Total before tax	1.1	0.9	2.3	1.4
Total reclassifications for the period, net of tax	0.8	0.6	1.8	0.9
Amortization of net loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	0.4	0.0	0.4	(0.6)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	0.4	0.0	1.1	0.0
Total reclassifications for the period, net of tax	4.4	(2.9)
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized gains on securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized (losses) gains on investments, excluding other-than-temporary impairment losses	5.1		14.5
Net realized (losses) gains on investments, excluding other-than-temporary impairment losses		(5.3)		(22.0)
Provision for income taxes	(1.1)	1.8	(3.1)	7.7
Net of tax	4.0	(3.5)	11.4	(14.3)
Reclassification out of Accumulated Other Comprehensive Income | Unamortized net (losses) on Pension Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision for income taxes	(0.3)	(0.3)
Total reclassifications for the period, net of tax	0.8	0.6
Reclassification out of Accumulated Other Comprehensive Income | Settlement gain
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	0.7	0.9	1.9	2.0
Interest Expense | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense	(0.5)	0.0	(1.4)	0.0
Provision for income taxes | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision for income taxes	$ (0.1)	$ 0.0	$ (0.3)	$ 0.0