Business Segments - Segment Assets (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total Assets	$ 13,364.0	$ 14,916.5
Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Total Assets	5,492.3	5,936.5
Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Total Assets	1,175.3	1,230.1
Life & Health Insurance
Segment Reporting Information [Line Items]
Total Assets	4,850.8	6,062.6
Corporate and Other, Net
Segment Reporting Information [Line Items]
Total Assets	$ 1,845.6	$ 1,687.3