Business Segments - Segment Net Operating Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	$ 69.7	$ (55.7)
Less: Net (Loss) Income attributable to Noncontrolling Interest	(1.1)	0.0
Net Income (Loss)	71.3	(80.1)
Operating Segments
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	81.1	(45.2)
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	69.2	(58.4)
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	5.0	(9.5)
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	11.9	13.2
Corporate and Other
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(12.5)	(10.5)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Change in Fair Value of Equity and Convertible Securities	2.7	1.3
Net Realized Investment Gains	5.2	5.1
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net of Tax	(1.2)	1.7
Acquisition and Disposition Related Transaction, Integration, Restructuring and Other Costs	$ (10.1)	$ (23.0)