Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	$ 1,166.9	$ 1,337.6	$ 2,347.8	$ 2,657.6
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	932.3	1,043.7	1,876.5	2,065.3
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	132.4	149.9	269.8	305.5
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	102.2	144.0	201.5	286.8
Operating Segments | Personal Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	766.6	905.8	1,554.5	1,807.5
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	165.7	137.9	322.0	257.8
Operating Segments | Preferred Automobile | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	75.7	94.0	153.9	190.0
Operating Segments | Homeowners | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	50.0	47.7	102.2	99.0
Operating Segments | Other Personal Lines | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	6.7	8.2	13.7	16.5
Operating Segments | Life | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	84.8	86.8	167.0	169.5
Operating Segments | Accident and Health | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	5.8	45.1	11.7	90.9
Operating Segments | Property | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	$ 11.6	$ 12.1	$ 22.8	$ 26.4