Other Comprehensive Income (Loss) (Reclassifications out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other	$ (26.5)	$ 4.6	$ 17.9
Income from continuing operations before income taxes	144.0	521.3	444.8
Loss adjustment expenses and other operating expenses	(3,623.4)	(3,134.2)	(2,845.2)
Income tax benefit (expense)	(27.2)	(101.3)	(82.8)
Income from continuing operations	116.8	420.0	362.0
Net income	116.0	418.7	358.7
Life [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from discontinued businesses, net of taxes	(0.8)	(2.5)	(3.7)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net income	(38.8)	3.3	1.2
Net Unrealized Appreciation (Depreciation) on Investments [Member] | 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	(26.7)	3.0	13.4
Recoveries (impairments) losses on investments	(18.5)	(1.3)	(17.6)
Income from continuing operations before income taxes	(45.2)	1.7	(4.2)
Income tax benefit (expense)	10.9	4.2	10.2
Net income	(34.5)	6.0	5.9
Net Unrealized Appreciation (Depreciation) on Investments [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accelerated Share Repurchase Program
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income from continuing operations	(34.3)	5.9	6.0
Net Unrealized Appreciation (Depreciation) on Investments [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Life [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from discontinued businesses, net of taxes	(0.2)	0.1	(0.1)
Defined Benefit Pension and Postretirement Plans [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Loss adjustment expenses and other operating expenses	(5.5)	(3.4)	(6.0)
Income tax benefit (expense)	1.2	0.7	1.3
Income from continuing operations	$ (4.3)	$ (2.7)	$ (4.7)