Business Segments - Segment Expenses (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]
Incurred Losses and LAE Related to Prior Years			$ 14.4	$ 17.0
Income Tax Expense (Benefit)	$ 17.7	$ 17.5	40.5	33.9
Operating Segments
Segment Reporting Information [Line Items]
Operating Expenses	1,117.0	940.2	2,164.7	1,883.5
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Total Incurred Losses and LAE	749.4	602.2	1,436.2	1,221.3
Policy Acquisition Costs	140.1	116.2	272.4	229.4
Business Unit Operating Costs	40.7	30.4	78.4	60.9
Corporate Overhead Costs	34.0	34.1	69.1	67.3
Total Insurance Expenses	214.8	180.7	419.9	357.6
Income Tax Expense (Benefit)	19.9	25.6	44.1	42.9
Operating Expenses	984.1	808.5	1,900.2	1,621.8
Operating Segments | Specialty Property & Casualty Insurance | Non-catastrophe Losses
Segment Reporting Information [Line Items]
Incurred Losses and LAE Related to Current Year	730.1	592.8	1,412.4	1,201.8
Incurred Losses and LAE Related to Prior Years	13.6	(0.8)	14.1	4.5
Operating Segments | Specialty Property & Casualty Insurance | Catastrophe Losses
Segment Reporting Information [Line Items]
Incurred Losses and LAE Related to Current Year	5.3	10.3	9.1	14.4
Incurred Losses and LAE Related to Prior Years	0.4	(0.1)	0.6	0.6
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Policyholders’ Benefits and Incurred Losses and LAE	63.5	63.9	125.7	126.9
Policy Acquisition Costs	35.2	34.2	64.9	65.4
Business Unit Operating Costs	24.0	24.2	51.5	47.5
Corporate Overhead Costs	8.5	10.6	17.7	21.0
Total Insurance Expenses	67.7	69.0	134.1	133.9
Income Tax Expense (Benefit)	1.7	(1.2)	4.7	0.9
Operating Expenses	$ 132.9	$ 131.7	$ 264.5	$ 261.7