Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,356.1	$ 1,206.5	$ 3,894.6	$ 3,458.2
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	1,028.3	871.4	2,916.2	2,452.9
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	163.7	174.5	489.1	519.0
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	164.1	160.6	489.3	486.3
Operating Segments | Specialty Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	920.6	792.2	2,615.6	2,235.2
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	107.7	79.2	300.6	217.7
Operating Segments | Personal Automobile | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	102.6	110.6	309.1	324.6
Operating Segments | Homeowners | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	52.5	55.0	154.6	167.4
Operating Segments | Other Personal Lines | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	8.6	8.9	25.4	27.0
Operating Segments | Life | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	101.5	96.3	300.2	289.2
Operating Segments | Accident and Health | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	47.0	48.9	142.3	149.1
Operating Segments | Property | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	$ 15.6	$ 15.4	$ 46.8	$ 48.0