Accumulated Other Comprehensive Income (Reclassification out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized (losses) gains on investments, excluding other-than-temporary impairment losses	$ (11.4)		$ (10.9)
Net realized (losses) gains on investments, excluding other-than-temporary impairment losses		$ 13.3		$ 17.1
Provision for income taxes	(11.3)	(58.5)	(41.8)	(129.8)
Net income	67.0	120.2	173.0	264.0
Interest expense	26.0	12.4	47.5	25.0
Total reclassifications for the period, net of tax	5.0	(6.5)	7.7	(10.5)
Net unrealized gains on securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period, net of tax	4.7	(7.0)	7.4	(10.8)
OTTI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period, net of tax	0.0	0.0	0.0	0.0
Unamortized net (losses) on Pension Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax			1.2	0.5
Provision for income taxes			0.2	0.2
Total reclassifications for the period, net of tax	1.0	0.5	1.0	0.3
Settlement gain
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax			0.0	(0.6)
Amortization of net loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax			1.2	1.1
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period, net of tax	5.0	(6.5)
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	6.0		9.4
Net realized (losses) gains on investments, excluding other-than-temporary impairment losses		(10.8)		(16.7)
Provision for income taxes	(1.3)	3.8	(2.0)	5.9
Net income	4.7	(7.0)	7.4	(10.8)
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision for income taxes	0.2	0.0	0.2	0.0
Interest expense	(0.9)	0.0	(0.9)	0.0
Net of tax	(0.7)	0.0	$ (0.7)	$ 0.0
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	(0.2)	(0.3)
Total reclassifications for the period, net of tax	1.0	0.5
Reclassification out of Accumulated Other Comprehensive Income | Amortization of net loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	$ 1.2	$ 0.8