Business Segments - Segment Expenses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Incurred Losses and LAE Related to Prior Years	$ (1.1)	$ 8.8
Income Tax Expense	22.8	16.4
Operating Segments
Segment Reporting Information [Line Items]
Operating Expenses	1,047.7	943.3
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Total Incurred Losses and LAE	686.8	619.1
Policy Acquisition Costs	132.3	113.2
Business Unit Operating Costs	37.7	30.5
Corporate Overhead Costs	35.1	33.2
Total Insurance Expenses	205.1	176.9
Income Tax Expense	24.2	17.3
Operating Expenses	916.1	813.3
Operating Segments | Specialty Property & Casualty Insurance | Non-catastrophe Losses
Segment Reporting Information [Line Items]
Incurred Losses and LAE Related to Current Year	682.3	609.0
Incurred Losses and LAE Related to Prior Years	0.5	5.3
Operating Segments | Specialty Property & Casualty Insurance | Catastrophe Losses
Segment Reporting Information [Line Items]
Incurred Losses and LAE Related to Current Year	3.8	4.1
Incurred Losses and LAE Related to Prior Years	0.2	0.7
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Policyholders’ Benefits and Incurred Losses and LAE	62.2	63.0
Policy Acquisition Costs	29.7	31.2
Business Unit Operating Costs	27.5	23.3
Corporate Overhead Costs	9.2	10.4
Total Insurance Expenses	66.4	64.9
Income Tax Expense	3.0	2.1
Operating Expenses	$ 131.6	$ 130.0