v3.26.3
Segment Information - Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Underwriting operations
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Segment Reporting [Line Items]        
Under- writing Margin 11.30% 10.50% 12.50% 12.70%
Combined Ratio 88.70% 89.50% 87.50% 87.30%
Operating Segments | Personal Lines        
Segment Reporting [Line Items]        
Under- writing Margin 12.00% 10.50% 12.80% 12.90%
Combined Ratio 88.00% 89.50% 87.20% 87.10%
Operating Segments | Commercial Lines        
Segment Reporting [Line Items]        
Under- writing Margin 7.40% 10.80% 11.00% 12.20%
Combined Ratio 92.60% 89.20% 89.00% 87.80%