Other Comprehensive Income (Reclassifications 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 gains (losses) from sales and other	$ (0.2)	$ 14.6	$ (0.1)	$ 25.0
Net other–than–temporary impairment losses on investments recognized in earnings	(0.4)	(1.3)	(2.8)	(4.5)
Income from continuing operations before income taxes	122.7	82.7	281.4	197.6
Loss adjustment expenses and other operating expenses	(1,043.9)	(1,003.2)	(3,110.1)	(2,977.8)
Income tax benefit (expense)	(18.7)	(23.2)	(44.5)	(53.4)
Continued Operations; Net of tax	104.0	59.5	236.9	144.2
Discontinued Operations; Net of tax	(3.6)		30.5
Net of tax	100.4	11.1	267.4	134.7
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	0.2	9.1	0.8	16.8
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other	(0.2)	14.7	1.6	24.8
Net other–than–temporary impairment losses on investments recognized in earnings	(0.4)	(1.3)	(1.7)	(3.3)
Income from continuing operations before income taxes	(0.6)	13.4	(0.1)	21.5
Income tax benefit (expense)	2.5	(2.4)	8.2	1.2
Continued Operations; Net of tax	1.9	11.0	8.1	22.7
Discontinued Operations; Net of tax	0.3	0.4	(1.5)	1.1
Net of tax	2.2	11.4	6.6	23.8
Reclassification out of Accumulated Other Comprehensive Income [Member] | Defined Benefit Pension and Postretirement Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss adjustment expenses and other operating expenses	(2.5)	(3.1)	(7.2)	(9.3)
Income tax benefit (expense)	0.5	1.1	1.5	3.3
Continued Operations; Net of tax	(2.0)	(2.0)	(5.7)	(6.0)
Discontinued Operations; Net of tax		(0.3)	(0.1)	(1.0)
Net of tax	$ (2.0)	$ (2.3)	$ (5.8)	$ (7.0)