v3.26.3
Accumulated Other Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Net realized losses on investments and fair value changes to equity securities $ 25.3 $ 13.6 $ 56.7 $ 51.3
Provision for income taxes 75.2 65.6 215.0 156.4
Interest expense 28.3 27.9 85.0 81.4
Loss on extinguishment of debt 0.0 1.3 0.0 1.3
Underwriting, selling, general and administrative expenses 2,368.4 2,161.5 6,956.1 6,366.5
Income before income tax expense (350.4) (331.2) (1,062.9) (803.9)
Net of tax (275.2) (265.6) (847.9) (647.5)
Reclassification out of Accumulated Other Comprehensive Income        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Net of tax 9.5 11.6 30.7 43.6
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized losses on investments        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Net realized losses on investments and fair value changes to equity securities 11.9 15.5 38.3 53.6
Provision for income taxes (2.5) (3.3) (8.0) (11.3)
Net of tax 9.4 12.2 30.3 42.3
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized (gains) losses on derivative transactions related to        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Provision for income taxes 0.1 0.2 0.3 0.3
Interest expense (0.6) (0.7) (1.7) (2.1)
Loss on extinguishment of debt 0.0 (0.3) 0.0 (0.3)
Underwriting, selling, general and administrative expenses 0.0 0.0 0.0 0.8
Income before income tax expense (0.6) (1.0) (1.7) (1.6)
Net of tax (0.5) (0.8) (1.4) (1.3)
Reclassification out of Accumulated Other Comprehensive Income | Unamortized net losses on Pension Plans        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Provision for income taxes (0.2) (0.1) (0.4) (0.8)
Income before income tax expense 0.8 0.3 2.2 3.4
Net of tax 0.6 0.2 1.8 2.6
Reclassification out of Accumulated Other Comprehensive Income | Amortization of net loss        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Net periodic benefit cost 0.8 0.3 2.2 0.9
Reclassification out of Accumulated Other Comprehensive Income | Settlement loss        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Net periodic benefit cost $ 0.0 $ 0.0 $ 0.0 $ 2.5