Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,307.0	$ 1,356.1	$ 3,999.3	$ 3,894.6
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	999.5	1,028.3	3,064.8	2,916.2
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	149.3	163.7	454.8	489.1
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	158.2	164.1	479.7	489.3
Operating Segments | Personal Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	858.8	920.6	2,666.3	2,615.6
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	140.7	107.7	398.5	300.6
Operating Segments | Personal Automobile | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	89.5	102.6	279.5	309.1
Operating Segments | Homeowners | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	51.9	52.5	150.9	154.6
Operating Segments | Other Personal Lines | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	7.9	8.6	24.4	25.4
Operating Segments | Life | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	100.4	101.5	304.6	300.2
Operating Segments | Accident and Health | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	45.9	47.0	136.8	142.3
Operating Segments | Property | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	$ 11.9	$ 15.6	$ 38.3	$ 46.8