Business Segments - Segment Net Operating Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	$ (65.2)	$ (51.6)
Loss from Early Extinguishment of Debt	0.0	(3.7)
Net Loss	(80.1)	(86.3)
Operating Segments
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(54.7)	(39.2)
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(58.4)	(44.7)
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(9.5)	(6.1)
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	13.2	11.6
Corporate and Other
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(10.5)	(12.4)
Other	(10.5)	(12.4)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Change in Fair Value of Equity and Convertible Securities	1.3	(22.3)
Net Realized Investment Gains	5.1	1.2
Impairment Gains (Losses)	1.7	(7.0)
Acquisition and Disposition Related Transaction, Integration, Restructuring and Other Costs	(23.0)	(3.7)
Loss from Early Extinguishment of Debt	$ 0.0	$ (2.9)