Other Comprehensive Income (Loss) (Reclassifications out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains from sales and other	$ 3.6	$ 8.2	$ 6.8	$ 12.1
Income from continuing operations before income taxes	42.5	148.6	316.5	237.8
Loss adjustment expenses and other operating expenses	(844.0)	(709.7)	(2,370.4)	(2,149.9)
Tax benefit	(7.7)	(29.1)	(59.3)	(41.7)
Income from continuing operations	34.8	119.5	257.2	196.1
Net income	34.0	118.9	255.2	194.1
Life [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Discontinued operations - life businesses	(0.8)	(0.6)	(2.0)	(2.0)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	2.3	6.7	6.1	(5.0)
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	(0.8)	(1.4)	(2.5)	(4.4)
Tax benefit	0.2	0.3	0.5	0.9
Income from continuing operations	(0.6)	(1.1)	(2.0)	(3.5)
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 from sales and other	2.5	5.0	5.7	8.9
Impairment recoveries (losses) on investments	0.1	1.7	(0.1)	(19.1)
Income from continuing operations before income taxes	2.6	6.7	5.6	(10.2)
Tax benefit	0.3	1.0	2.5	9.0
Income from continuing operations	2.9	7.7	8.1	(1.2)
Net income	$ 2.9	7.8	$ 8.1	(1.5)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member] | Life [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Discontinued operations - life businesses		$ 0.1		$ (0.3)