Business Segments - Segment Operating Profit (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 163.0	$ 95.9	$ 396.0	$ 211.2
Operating Costs and Expenses [Abstract]
Income from Change in Fair Value of Equity and Convertible Securities	9.8	11.0	99.7	12.1
Net Impairment Losses Recognized in Earnings	(1.8)	(1.8)	(12.1)	(2.3)
Loss from Early Extinguishment of Debt	(5.8)	0.0	(5.8)	0.0
Income from Continuing Operations before Income Taxes	161.5	80.5	504.7	193.0
Operating Segments
Segment Reporting Information [Line Items]
Operating Income (Loss)	166.1	74.5	400.0	203.7
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	26.5	14.1	36.2	22.0
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	98.6	26.9	277.4	84.1
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	41.0	33.5	86.4	97.6
Corporate and Other
Segment Reporting Information [Line Items]
Operating Income (Loss)	(3.1)	21.4	(4.0)	7.5
Operating Costs and Expenses [Abstract]
Partial Satisfaction of Judgment	0.0	35.7	20.1	35.7
Other	(3.1)	(14.3)	(24.1)	(28.2)
Segment Reconciling Items
Operating Costs and Expenses [Abstract]
Income from Change in Fair Value of Equity and Convertible Securities	9.8	11.0	99.7	12.1
Net Realized Gains on Sales of Investments	1.7	3.6	39.1	10.0
Net Impairment Losses Recognized in Earnings	(1.8)	(1.8)	(12.1)	(2.3)
Acquisition Related Transaction, Integration and Other Costs	$ (5.4)	$ (28.2)	$ (12.2)	$ (38.0)