Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	$ 1,180.9	$ 1,320.0
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	944.2	1,021.6
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	137.4	155.6
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	99.3	142.8
Operating Segments | Personal Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	787.9	901.7
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	156.3	119.9
Operating Segments | Preferred Automobile | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	78.2	96.0
Operating Segments | Homeowners | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	52.2	51.3
Operating Segments | Other Personal Lines | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	7.0	8.3
Operating Segments | Life | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	82.2	82.7
Operating Segments | Accident and Health | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	5.9	45.8
Operating Segments | Property | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	$ 11.2	$ 14.3