v3.26.1
Accumulated Other Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Net realized losses on investments and fair value changes to equity securities $ 10.2 $ 21.7 $ 31.4 $ 37.7
Provision for income taxes 78.3 53.7 139.8 90.8
Interest expense 28.4 26.7 56.7 53.5
Underwriting, selling, general and administrative expenses 2,300.6 2,121.2 4,587.7 4,205.0
Income before income tax expense (376.9) (289.0) (712.5) (472.7)
Net of tax (298.6) (235.3) (572.7) (381.9)
Reclassification out of Accumulated Other Comprehensive Income        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Net of tax 8.9 16.7 21.2 32.0
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 11.1 21.1 26.4 38.1
Provision for income taxes (2.3) (4.4) (5.5) (8.0)
Net of tax 8.8 16.7 20.9 30.1
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 0.0 0.2 0.1
Interest expense (0.6) (0.7) (1.1) (1.4)
Underwriting, selling, general and administrative expenses 0.0 0.5 0.0 0.8
Income before income tax expense (0.6) (0.2) (1.1) (0.6)
Net of tax (0.5) (0.2) (0.9) (0.5)
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 (0.1) (0.1) (0.2) (0.7)
Income before income tax expense 0.7 0.3 1.4 3.1
Net of tax 0.6 0.2 1.2 2.4
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 0.7 0.3 1.4 0.6
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