Other Comprehensive Income (Reclassifications out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains from sales and other	$ 4.3	$ 12.2	$ 31.0	$ 38.8
Loss adjustment expenses and other operating expenses	(1,117.1)	(1,123.1)	(3,465.7)	(3,489.3)
Net other-than-temporary impairment losses on investments recognized in earnings	(0.1)	(4.2)	(26.0)	(8.8)
Total before tax	124.1	110.4	225.1	340.0
Tax benefit (expense)	(35.8)	(33.2)	(56.8)	(87.0)
Net of tax	88.4	78.3	168.6	253.9
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	5.7	2.9	12.3	17.9
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized Appreciation (Depreciation) on Investments:
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains from sales and other	3.4	5.9	30.7	32.3
Net other-than-temporary impairment losses on investments recognized in earnings	(0.1)	(4.2)	(26.0)	(8.8)
Total before tax	3.3	1.7	4.7	23.5
Tax benefit (expense)	3.9	2.9	12.4	1.2
Net of tax	7.2	4.6	17.1	24.7
Reclassification out of Accumulated Other Comprehensive Income [Member] | Defined Benefit Pension and Postretirement Plans:
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss adjustment expenses and other operating expenses	(2.5)	(2.7)	(7.5)	(10.0)
Tax benefit (expense)	1.0	1.0	2.7	3.2
Net of tax	$ (1.5)	$ (1.7)	$ (4.8)	$ (6.8)