Business Segments - Segment Net Operating Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Total Revenues	$ 1,193.0	$ 1,143.0
Total Expenses	1,073.2	1,056.4
Less: Net Loss attributable to Noncontrolling Interest	(2.7)	(1.1)
Income before Income Taxes	99.7	71.3
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,162.8	1,024.4
Total Segment Adjusted Net Operating Income	115.1	81.1
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Total Revenues	1,014.0	882.5
Total Expenses	(916.1)	(813.3)
Operating Income (Loss), Net of Tax	97.9	69.2
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Total Revenues	148.8	141.9
Total Expenses	(131.6)	(130.0)
Operating Income (Loss), Net of Tax	17.2	11.9
Corporate and Other
Segment Reporting Information [Line Items]
Corporate and Other Adjusted Net Operating Loss	(11.4)	(12.5)
Non-Core Operations
Segment Reporting Information [Line Items]
Total Revenues	27.9	106.2
Operating Income (Loss), Net of Tax	(3.9)	5.0
Change in Fair Value of Equity and Convertible Securities	0.1	2.7
Net Realized Investment Gains	0.7	5.2
Impairment Losses	0.2	(1.2)
Acquisition and Disposition Related Transaction, Integration, Restructuring and Other Costs	(4.2)	(10.1)
Debt Extinguishment, Pension Settlement and Other Charges	$ 0.4	$ 0.0