Other Comprehensive Income (Loss) (Reclassifications out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other	$ (8.9)	$ (26.5)	$ 4.6
Income from continuing operations before income taxes	41.1	144.0	521.3
Loss adjustment expenses and other operating expenses	(4,134.6)	(3,623.4)	(3,134.2)
Income tax benefit	(7.6)	(27.2)	(101.3)
Income from continuing operations	33.5	116.8	420.0
Net income	35.3	116.0	422.8
Life [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from discontinued businesses, net of taxes	0.6	(0.8)	1.6
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net income	(19.5)	(38.8)	7.4
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	(9.1)	(26.7)	3.0
Impairments losses on investments	(11.1)	(18.5)	(1.3)
Income from continuing operations before income taxes	(20.2)	(45.2)	1.7
Income tax benefit	5.6	10.9	4.2
Net income	(14.6)	(34.5)	6.0
Net Unrealized Appreciation (Depreciation) on Investments [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accelerated Share Repurchase Agreement [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income from continuing operations	(14.6)	(34.3)	5.9
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
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	(7.9)	(5.5)	(3.4)
Income tax benefit	1.7	1.2	0.7
Income from continuing operations	(6.2)	$ (4.3)	(2.7)
Net change in market risk | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income tax benefit	(0.3)		(1.0)
Income (loss) from discontinued businesses, net of taxes	1.3		4.1
Net change in market risk | 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	$ 1.6		$ 5.1