Other Comprehensive Income (Reclassifications out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other	$ (0.4)	$ 0.6
Net other–than–temporary impairment losses on investments recognized in earnings		(0.6)
Income from continuing operations before income taxes	148.6	59.1
Loss adjustment expenses and other operating expenses	(1,070.9)	(1,032.4)
Tax benefit	(26.0)	(8.6)
Continued operations; net of tax	122.6	50.5
Net of tax	122.4	67.7
Chaucer [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss adjustment expenses and other operating expenses		(216.5)
Discontinued operations - Chaucer business	(0.3)	17.3
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	1.3	(0.1)
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	(0.3)
Net other–than–temporary impairment losses on investments recognized in earnings		(0.6)
Income from continuing operations before income taxes	(0.3)	(0.6)
Tax benefit	3.9	2.7
Continued operations; net of tax	3.6	2.1
Net of tax	3.6	1.9
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member] | Chaucer [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Discontinued operations - Chaucer business		(0.2)
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	(2.9)	(2.5)
Tax benefit	0.6	0.5
Continued operations; net of tax	$ (2.3)	$ (2.0)