Business Segments - Segment Net Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Revenues	$ 4,638.6	$ 4,944.2	$ 5,523.9
Less: Net Loss attributable to Noncontrolling Interest	(5.3)	(0.2)	0.0
Debt Extinguishment, Pension Settlement, and Other Charges			(2.9)
Goodwill Impairment Charges	0.0	(49.6)	0.0
Net Income (Loss)	317.8	(272.1)	(286.6)
Operating Segments
Segment Reporting Information [Line Items]
Revenues	4,337.7	4,388.4	4,965.3
Segment Net Operating Income (Loss)	426.5	(5.3)	(78.6)
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Revenues	3,772.1	3,806.9	4,183.2
Expenses	(3,395.8)	(3,864.0)	(4,330.6)
Segment Net Operating Income (Loss)	376.3	(57.1)	(147.4)
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Revenues	565.6	581.5	782.1
Expenses	(515.4)	(529.7)	(713.3)
Segment Net Operating Income (Loss)	50.2	51.8	68.8
Corporate and Other
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(50.3)	(42.1)	(37.8)
Non-Core Operations
Segment Reporting Information [Line Items]
Revenues	282.4	558.4	640.5
Segment Net Operating Income (Loss)	(28.2)	(17.0)	(25.9)
Change in Fair Value of Equity and Convertible Securities	(2.1)	3.7	(63.1)
Net Realized Investment Gains (Losses)	10.4	(14.7)	3.4
Impairment Losses	(4.6)	(0.9)	(20.4)
Acquisition and Disposition Related Transaction, Integration, Restructuring and Other Costs	(31.8)	(95.0)	(61.3)
Debt Extinguishment, Pension Settlement, and Other Charges	(7.4)	(55.5)
Goodwill Impairment Charges	$ 0.0	$ (45.5)	$ 0.0