Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	$ 1,117.8	$ 1,290.9	$ 3,465.6	$ 3,948.5
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	890.4	999.5	2,766.9	3,064.8
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	102.1	142.1	303.6	428.9
Operating Segments | Specialty and Life Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	992.5	1,141.6	3,070.5	3,493.7
Operating Segments | Preferred Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	125.3	149.3	395.1	454.8
Operating Segments | Personal Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	724.0	858.8	2,278.5	2,666.3
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	166.4	140.7	488.4	398.5
Operating Segments | Life | Life Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	84.9	84.3	251.9	253.8
Operating Segments | Accident and Health | Life Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	5.8	45.9	17.5	136.8
Operating Segments | Property | Life Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	$ 11.4	$ 11.9	$ 34.2	$ 38.3