Business Segments - Segment Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total Assets	$ 14,341.9	$ 12,989.1
Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Total Assets	4,897.1	4,435.2
Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Total Assets	1,711.2	1,549.8
Life & Health Insurance
Segment Reporting Information [Line Items]
Total Assets	6,457.0	5,847.9
Corporate and Other, Net
Segment Reporting Information [Line Items]
Total Assets	$ 1,276.6	$ 1,156.2