Business Segments - Segment Operating Profit (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 132.4		$ (38.9)	$ 332.8	$ (131.9)
Operating Costs and Expenses [Abstract]
Less: Net Loss attributable to Noncontrolling Interest	(1.8)		(0.1)	(4.5)	(0.1)
Change in Fair Value of Equity and Convertible Securities	(2.3)		2.8	(0.1)	6.9
Net Realized Investment Gains (Losses)				9.2	(38.3)
Impairment Losses	(2.2)		(1.1)	(3.8)	0.1
Debt Extinguishment, Pension Settlement and Other Charges	(2.8)	$ (70.2)		(0.1)	(70.2)
Goodwill Impairment Charge	0.0		0.0	0.0	(49.6)
Income (Loss) before Income Taxes	92.5		(190.7)	273.7	(410.5)
Operating Segments
Segment Reporting Information [Line Items]
Operating Income (Loss)	147.8		(25.7)	374.8	(90.5)
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	129.8		(43.2)	344.2	(132.9)
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	18.0		17.5	30.6	42.4
Operating Segments | Non-Core Operations
Segment Reporting Information [Line Items]
Operating Income (Loss)	(22.2)		(9.6)	(33.5)	(25.5)
Corporate and Other
Segment Reporting Information [Line Items]
Operating Income (Loss)	(17.2)		(13.3)	(46.5)	(41.5)
Segment Reconciling Items
Operating Costs and Expenses [Abstract]
Change in Fair Value of Equity and Convertible Securities	(2.3)		2.8	(0.1)	6.9
Net Realized Investment Gains (Losses)	1.1		(30.3)	9.2	(38.3)
Impairment Losses	2.2		1.1	(3.8)	0.1
Acquisition and Disposition Related Transaction, Integration, Restructuring and Other Costs	(11.5)		(43.4)	(30.8)	(102.0)
Debt Extinguishment, Pension Settlement and Other Charges	(2.2)		(55.5)	(0.1)	(55.5)
Goodwill Impairment Charge	$ 0.0		$ 0.0	$ 0.0	$ (45.5)