v3.26.3
Segment Information - Schedule of Return on Equity (Details)
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Segment Reporting [Line Items]        
ROE 15.10% 14.00% 13.80% 13.00%
Operating Segments | Insurance Operations        
Segment Reporting [Line Items]        
ROE 3.50% 1.60% 2.60% 2.00%
Operating Segments | Standard Commercial Lines        
Segment Reporting [Line Items]        
ROE 1.80% (1.00%) 0.70% (0.10%)
Operating Segments | Standard Personal Lines        
Segment Reporting [Line Items]        
ROE (0.30%) (1.00%) 0.30% 0.00%
Operating Segments | E&S Lines        
Segment Reporting [Line Items]        
ROE 2.00% 3.60% 1.60% 2.10%
Operating Segments | Investments        
Segment Reporting [Line Items]        
Net investment income earned 15.20% 13.60% 14.20% 13.20%
Net realized and unrealized investment gains (losses) (1.80%) 0.80% (0.50%) 0.40%
ROE 13.40% 14.40% 13.70% 13.60%
Reconciling Item, Corporate        
Segment Reporting [Line Items]        
ROE (1.80%) (2.00%) (2.50%) (2.60%)