Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,068.5	$ 1,117.8	$ 3,134.1	$ 3,465.6
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	919.0	890.4	2,621.6	2,766.9
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	100.6	102.1	298.7	303.6
Operating Segments | Specialty and Life Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	1,019.6	992.5	2,920.3	3,070.5
Operating Segments | Non-Core Operations
Segment Reporting Information [Line Items]
Premiums Earned, Net	48.9	125.3	213.8	395.1
Operating Segments | Personal Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	731.3	724.0	2,098.1	2,278.5
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	187.7	166.4	523.5	488.4
Operating Segments | Life | Life Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	84.2	84.9	249.2	251.9
Operating Segments | Accident and Health | Life Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	5.6	5.8	16.8	17.5
Operating Segments | Property | Life Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 10.8	$ 11.4	$ 32.7	$ 34.2