Other Comprehensive Income (Reclassifications out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains from sales and other	$ 14.7	$ 4.3	$ 25.7	$ 31.0
Net other–than–temporary impairment losses on investments recognized in earnings	(1.3)	(0.1)	(4.5)	(26.0)
Income before income taxes	8.9	124.1	178.9	225.1
Loss adjustment expenses and other operating expenses	(1,316.3)	(1,117.1)	(3,673.3)	(3,465.7)
Tax (expense) benefit	3.4	(35.8)	(43.0)	(56.8)
Net of tax	11.1	88.4	134.7	168.6
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	9.1	5.7	16.8	12.3
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized Appreciation on Investments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains from sales and other	15.2	3.4	26.5	30.7
Net other–than–temporary impairment losses on investments recognized in earnings	(1.3)	(0.1)	(3.3)	(26.0)
Income before income taxes	13.9	3.3	23.2	4.7
Tax (expense) benefit	(2.5)	3.9	0.6	12.4
Net of tax	11.4	7.2	23.8	17.1
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	(3.5)	(2.5)	(10.5)	(7.5)
Tax (expense) benefit	1.2	1.0	3.5	2.7
Net of tax	$ (2.3)	$ (1.5)	$ (7.0)	$ (4.8)