Business Segments - Segment Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Prior Years	$ 22.6		$ (14.6)
Policy Acquisition Costs	641.6	$ 707.6	838.5
Business Unit Operating Costs	277.4	256.1	282.4
Corporate Overhead Costs	194.9	200.0	207.8
Insurance Expenses	1,113.9	1,163.7	1,328.7
Income Tax Expense (Benefit)	76.0	(74.8)	(84.4)
Total Expenses	4,250.1	5,291.3	5,894.9
Operating Segments
Segment Reporting Information [Line Items]
Total Expenses	3,911.2	4,393.7	5,043.9
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Total Incurred Losses and LAE	2,541.7	3,141.9	3,578.2
Policy Acquisition Costs	478.7	496.4	577.8
Business Unit Operating Costs	145.0	107.8	81.2
Corporate Overhead Costs	135.8	137.1	142.9
Insurance Expenses	759.5	741.3	801.9
Income Tax Expense (Benefit)	94.6	(19.2)	(49.5)
Total Expenses	3,395.8	3,864.0	4,330.6
Operating Segments | Specialty Property & Casualty Insurance | Non-catastrophe Losses
Segment Reporting Information [Line Items]
Current Year	2,514.8	2,974.5	3,569.2
Prior Years	6.3	135.2	(14.6)
Operating Segments | Specialty Property & Casualty Insurance | Catastrophe Losses
Segment Reporting Information [Line Items]
Current Year	19.9	34.5	23.0
Prior Years	0.7	(2.3)	0.6
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Policy Acquisition Costs	133.9	140.0	165.1
Business Unit Operating Costs	96.9	96.7	137.2
Corporate Overhead Costs	41.3	39.1	41.0
Policyholders’ Benefits and Incurred Losses and LAE	234.5	243.4	360.8
Insurance Expenses	272.1	275.8	343.3
Income Tax Expense (Benefit)	8.8	10.5	9.2
Total Expenses	$ 515.4	$ 529.7	$ 713.3