Other Comprehensive Income (Reclassifications 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 gains (losses) from sales and other	$ (0.4)	$ 7.7	$ (0.1)	$ 11.0
Net other–than–temporary impairment losses on investments recognized in earnings	(1.9)	(1.8)	(2.6)	(3.2)
Income before income taxes	119.9	111.0	200.5	170.0
Loss adjustment expenses and other operating expenses	(1,242.9)	(1,155.1)	(2,493.2)	(2,357.0)
Tax benefit	(20.7)	(32.6)	(33.5)	(46.4)
Net of tax	99.3	78.4	167.0	123.6
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	1.6	4.3	1.5	7.7
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		7.2	(0.2)	11.3
Net other–than–temporary impairment losses on investments recognized in earnings	0.1	(1.8)	(0.4)	(2.0)
Income before income taxes	0.1	5.4	(0.6)	9.3
Tax benefit	3.3	1.3	5.9	3.1
Net of tax	3.4	6.7	5.3	12.4
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	(2.3)	(3.5)	(4.8)	(7.0)
Tax benefit	0.5	1.1	1.0	2.3
Net of tax	$ (1.8)	$ (2.4)	$ (3.8)	$ (4.7)