Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,135.2	$ 1,052.9	$ 3,326.6	$ 2,320.8
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	191.0	184.0	565.1	541.9
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	783.4	711.2	2,278.7	1,309.6
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	160.8	157.7	482.8	469.3
Operating Segments | Personal Automobile | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	119.7	111.4	353.0	325.0
Operating Segments | Homeowners | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	61.5	62.5	182.6	186.5
Operating Segments | Other Personal Lines | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	9.8	10.1	29.5	30.4
Operating Segments | Specialty Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	719.2	655.3	2,092.5	1,229.0
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	64.2	55.9	186.2	80.6
Operating Segments | Life | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	96.2	95.2	289.0	284.3
Operating Segments | Accident and Health | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	47.6	44.9	142.4	132.0
Operating Segments | Property | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	$ 17.0	$ 17.6	$ 51.4	$ 53.0