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Accumulated Other Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]    
Net realized losses on investments and fair value changes to equity securities $ 21.2 $ 16.0
Provision for income taxes 61.5 37.1
Interest expense 28.3 26.8
Underwriting, selling, general and administrative expenses 2,287.1 2,083.8
Income before income tax expense (335.6) (183.7)
Net of tax (274.1) (146.6)
Reclassification out of Accumulated Other Comprehensive Income    
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]    
Net of tax 12.3 15.3
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 15.3 17.0
Provision for income taxes (3.2) (3.6)
Net of tax 12.1 13.4
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 0.1
Interest expense (0.5) (0.7)
Underwriting, selling, general and administrative expenses 0.0 0.3
Income before income tax expense (0.5) (0.4)
Net of tax (0.4) (0.3)
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 (0.1) (0.6)
Income before income tax expense 0.7 2.8
Net of tax 0.6 2.2
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
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 $ 2.5