Business Segments - Segment Assets (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total Assets	$ 14,916.5	$ 14,341.9
Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Total Assets	5,936.5	4,897.1
Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Total Assets	1,230.1	1,711.2
Life & Health Insurance
Segment Reporting Information [Line Items]
Total Assets	6,062.6	6,457.0
Corporate and Other, Net
Segment Reporting Information [Line Items]
Total Assets	$ 1,687.3	$ 1,276.6