Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,353.7	$ 1,337.7	$ 2,692.3	$ 2,538.5
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	1,043.7	1,010.3	2,065.3	1,887.9
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	149.9	163.2	305.5	325.4
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	160.1	164.2	321.5	325.2
Operating Segments | Personal Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	905.8	909.6	1,807.5	1,695.0
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	137.9	100.7	257.8	192.9
Operating Segments | Personal Automobile | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	94.0	103.5	190.0	206.5
Operating Segments | Homeowners | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	47.7	51.3	99.0	102.1
Operating Segments | Other Personal Lines | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	8.2	8.4	16.5	16.8
Operating Segments | Life | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	102.9	100.6	204.2	198.7
Operating Segments | Accident and Health | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	45.1	47.9	90.9	95.3
Operating Segments | Property | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	$ 12.1	$ 15.7	$ 26.4	$ 31.2