Business Segments - Segment Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total Assets	$ 12,989.1	$ 11,544.9
Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Total Assets	4,435.2	3,541.0
Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Total Assets	1,549.8	1,567.7
Life & Health Insurance
Segment Reporting Information [Line Items]
Total Assets	5,847.9	5,117.2
Corporate and Other, Net
Segment Reporting Information [Line Items]
Total Assets	$ 1,156.2	$ 1,319.0