Business Segments - Segment Assets (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Total Assets	$ 12,630.4	$ 12,742.7	$ 13,313.6
Operating Segments
Segment Reporting Information [Line Items]
Total Assets	11,084.6	11,044.0	11,543.3
Corporate and Other
Segment Reporting Information [Line Items]
Total Assets	774.7	623.7	545.4
Non-Core Operations
Segment Reporting Information [Line Items]
Total Assets	771.1	1,075.0	1,224.9
Specialty Property & Casualty Insurance | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	6,352.9	6,145.9	6,535.3
Specialty Property & Casualty Insurance | Operating Segments | Variable Interest Entity, Primary Beneficiary
Segment Reporting Information [Line Items]
Total Assets	41.5	4.6
Life Insurance | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	$ 4,731.7	$ 4,898.1	$ 5,008.0