Other Comprehensive Income (Loss) (Reclassifications out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other	$ 3.0	$ (1.6)
Income from continuing operations before income taxes	130.0	114.1
Loss adjustment expenses and other operating expenses	(787.5)	(781.3)
Tax benefit (expense)	(24.7)	(21.3)
Income from continuing operations	105.3	92.8
Net income	104.8	92.7
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other	3.0	(1.6)
Impairment losses on investments	(0.9)
Tax benefit (expense)	(0.4)	2.2
Net income	0.6	(0.1)
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	(1.4)	(0.8)
Tax benefit (expense)	0.3	0.1
Income from continuing operations	(1.1)	(0.7)
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]
Income from continuing operations before income taxes	2.1	(1.6)
Income from continuing operations	$ 1.7	$ 0.6