Business Segments - Segment Net Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	$ (47.2)	$ (116.4)		$ (209.5)
Debt Extinguishment, Pension Settlement, and Other Charges		(2.9)		0.0
Net Income (Loss)	(272.1)	(286.6)		(123.7)
Goodwill, Impairment Loss	(49.6)	0.0	[1]	0.0	[2]
Debt Extinguishment, Pension Settlement, and Other Charges	(70.2)
Operating Segments
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(5.3)	(78.6)		(171.1)
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(57.1)	(147.4)		(196.1)
Operating Segments | Non-Core Operations
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(17.0)	(25.9)		(12.5)
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	51.8	68.8		25.0
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(42.1)	(37.8)		(38.4)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Change in Fair Value of Equity and Convertible Securities	3.7	(63.1)		90.5
Net Realized Investment (Losses) Gains	(14.7)	3.4		51.2
Impairment Losses	(0.9)	(20.4)		(8.7)
Acquisition and Disposition Related Transaction, Integration, Restructuring and Other Costs	(95.0)	(61.3)		(34.7)
Goodwill, Impairment Loss	(45.5)	$ 0.0		$ 0.0
Debt Extinguishment, Pension Settlement, and Other Charges	$ 55.5

[1] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.

[2] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.