v3.26.1
Business Segments - Segment Assets (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Segment Reporting [Line Items]          
Total Assets $ 11,926.3 $ 12,472.7 $ 11,926.3 $ 12,472.7 $ 12,472.7
Goodwill Impairment 460.0 0.0 460.0 0.0  
Operating Segments          
Segment Reporting [Line Items]          
Total Assets 11,305.3 11,772.3 11,305.3 11,772.3  
Corporate and Other          
Segment Reporting [Line Items]          
Total Assets 265.9 332.3 265.9 332.3  
Non-Core Operations          
Segment Reporting [Line Items]          
Total Assets 355.1 368.1 355.1 368.1  
Goodwill Impairment (460.0) 0.0 (460.0) 0.0  
Specialty Property & Casualty Insurance | Operating Segments          
Segment Reporting [Line Items]          
Total Assets 6,525.1 6,932.8 6,525.1 6,932.8  
Specialty Property & Casualty Insurance | Operating Segments | Variable Interest Entity, Primary Beneficiary          
Segment Reporting [Line Items]          
Total Assets 66.6 74.5 66.6 74.5  
Life Insurance | Operating Segments          
Segment Reporting [Line Items]          
Total Assets $ 4,780.2 $ 4,839.5 $ 4,780.2 $ 4,839.5