Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,337.7	$ 1,085.3	$ 2,538.5	$ 2,251.7
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	1,010.3	759.0	1,887.9	1,581.5
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	163.2	163.6	325.4	344.5
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	164.2	162.7	325.2	325.7
Operating Segments | Specialty Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	909.6	689.8	1,695.0	1,443.0
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	100.7	69.2	192.9	138.5
Operating Segments | Personal Automobile | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	103.5	99.1	206.5	214.0
Operating Segments | Homeowners | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	51.3	55.6	102.1	112.4
Operating Segments | Other Personal Lines | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	8.4	8.9	16.8	18.1
Operating Segments | Life | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	100.6	95.7	198.7	192.9
Operating Segments | Accident and Health | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	47.9	50.8	95.3	100.2
Operating Segments | Property | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	$ 15.7	$ 16.2	$ 31.2	$ 32.6