Business Segments - Segment Net Operating Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting Information [Line Items]
Total Revenues	$ 1,225.6	$ 1,129.9	$ 2,418.6	$ 2,272.9
Total Expenses	1,138.1	1,038.0	2,211.3	2,094.4
Less: Net Loss attributable to Noncontrolling Interest	(2.8)	(1.0)	(5.5)	(2.1)
Income before Income Taxes	72.6	75.4	172.3	146.7
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,208.6	1,042.3	2,371.4	2,066.7
Total Segment Adjusted Net Operating Income	91.6	102.1	206.7	183.2
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Total Revenues	1,063.1	910.8	2,077.1	1,793.3
Total Expenses	(984.1)	(808.5)	(1,900.2)	(1,621.8)
Operating Income (Loss), Net of Tax	79.0	102.3	176.9	171.5
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Total Revenues	145.5	131.5	294.3	273.4
Total Expenses	(132.9)	(131.7)	(264.5)	(261.7)
Operating Income (Loss), Net of Tax	12.6	(0.2)	29.8	11.7
Corporate and Other
Segment Reporting Information [Line Items]
Corporate and Other Adjusted Net Operating Loss	(10.3)	(11.4)	(21.7)	(23.9)
Non-Core Operations
Segment Reporting Information [Line Items]
Total Revenues	21.3	83.8	49.2	190.0
Operating Income (Loss), Net of Tax	(8.3)	(13.4)	(4.4)	(8.4)
Change in Fair Value of Equity and Convertible Securities	(0.4)	(1.0)	(0.3)	1.7
Net Realized Investment (Losses) Gains	(0.1)	1.2	0.6	6.4
Impairment Losses	(2.8)	(0.1)	(2.6)	(1.3)
Acquisition and Disposition Related Transaction, Integration, Restructuring and Other Costs	(3.8)	(5.1)	(8.0)	(15.2)
Debt Extinguishment, Pension Settlement and Other Charges	$ 0.4	$ 2.1	$ 0.0	$ 2.1