Business Segments - Segment Operating Profit (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ (85.0)	$ (71.7)
Corporate and Other Operating Loss From:
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	1.7	(28.2)
Debt and Equity Securities, Realized Gain (Loss)	6.4	1.5
Impairment Gains (Losses)	2.1	(8.9)
Loss from Early Extinguishment of Debt	0.0	(3.7)
Loss before Income Taxes	(103.9)	(115.7)
Operating Segments
Segment Reporting Information [Line Items]
Operating Income (Loss)	(72.1)	(57.7)
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	(74.7)	(60.2)
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	(12.2)	(9.5)
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	14.8	12.0
Corporate and Other
Segment Reporting Information [Line Items]
Operating Income (Loss)	(12.9)	(14.0)
Corporate and Other Operating Loss From:
Other	(12.9)	(14.0)
Segment Reconciling Items
Corporate and Other Operating Loss From:
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	1.7	(28.2)
Debt and Equity Securities, Realized Gain (Loss)	6.4	1.5
Impairment Gains (Losses)	2.1	(8.9)
Acquisition and Disposition Related Transaction, Integration, Restructuring and Other Costs	(29.1)	(4.7)
Loss from Early Extinguishment of Debt	$ 0.0	$ (2.9)