Other Comprehensive Income (Loss) (Reclassifications out of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized losses from sales and other	$ (1.3)	$ (1.1)
Income (loss) before income taxes	145.3	(17.5)
Loss adjustment expenses and other operating expenses	(935.2)	(1,017.4)
Income tax benefit (expense)	(29.8)	5.5
Net income (loss)	115.5	(12.0)
Reclassification out of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income (loss)	(1.8)	(10.1)
Reclassification out of Accumulated Other Comprehensive Loss [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	(1.7)	(2.0)
Income tax benefit (expense)	0.3	0.4
Income (loss) from continuing operations	(1.4)	(1.6)
Reclassification out of Accumulated Other Comprehensive Loss [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized losses from sales and other	(1.5)	(1.1)
Recoveries (impairments) on investments	0.3	(10.7)
Income (loss) before income taxes	(1.2)	(11.8)
Income tax benefit (expense)	(0.6)	(3.3)
Income (loss) from continuing operations	(0.6)	(8.5)
Reclassification out of Accumulated Other Comprehensive Loss [Member] | Net change in market risk
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax benefit (expense)	(0.1)	0.0
Income (loss) from discontinued businesses, net of taxes	0.2	0.0
Reclassification out of Accumulated Other Comprehensive Loss [Member] | Net change in market risk | Life [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from discontinued businesses, net of taxes	$ 0.3	$ 0.0