Business Segments - Segment Net Operating Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	$ (16.9)	$ (37.2)	$ (82.1)	$ (88.8)
Loss from Early Extinguishment of Debt	0.0	0.0	0.0	(3.7)
Goodwill, pre tax impairment charge	49.6	0.0	49.6	0.0
Net Loss	(97.1)	(72.2)	(177.2)	(158.5)
Operating Segments
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(4.6)	(35.5)	(59.3)	(74.7)
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(10.8)	(38.9)	(69.2)	(83.6)
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(2.7)	(16.8)	(12.2)	(22.9)
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	8.9	20.2	22.1	31.8
Corporate and Other
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(12.3)	(1.7)	(22.8)	(14.1)
Other	(12.3)	(1.7)	(22.8)	(14.1)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Change in Fair Value of Equity and Convertible Securities	1.9	(32.0)	3.2	(54.3)
Net Realized Investment (Losses) Gains	(12.5)	8.7	(7.4)	9.9
Impairment Gains (Losses)	(0.8)	(3.9)	0.9	(10.9)
Acquisition and Disposition Related Transaction, Integration, Restructuring and Other Costs	(23.3)	(7.8)	(46.3)	(11.5)
Loss from Early Extinguishment of Debt	0.0	0.0	0.0	(2.9)
Goodwill, pre tax impairment charge	$ (45.5)	$ 0.0	$ (45.5)	$ 0.0