Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,206.5	$ 1,135.2	$ 3,458.2	$ 3,326.6
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	871.4	783.4	2,452.9	2,278.7
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	174.5	191.0	519.0	565.1
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	160.6	160.8	486.3	482.8
Operating Segments | Specialty Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	792.2	719.2	2,235.2	2,092.5
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	79.2	64.2	217.7	186.2
Operating Segments | Personal Automobile | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	110.6	119.7	324.6	353.0
Operating Segments | Homeowners | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	55.0	61.5	167.4	182.6
Operating Segments | Other Personal Lines | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	8.9	9.8	27.0	29.5
Operating Segments | Life | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	96.3	96.2	289.2	289.0
Operating Segments | Accident and Health | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	48.9	47.6	149.1	142.4
Operating Segments | Property | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	$ 15.4	$ 17.0	$ 48.0	$ 51.4