v3.22.2.2
Accumulated Other Comprehensive Income - Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2022
Sep. 30, 2021
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Underwriting, selling, general and administrative expenses $ 1,842.5 $ 1,784.0 $ 5,444.8 $ 5,210.4
Provision for income taxes 1.2 37.2 45.1 133.8
Net of tax (7.3) (879.8) (208.5) (1,240.9)
Net realized losses (gains) on investments 27.4 (112.1) 166.2 (123.2)
Interest expense 26.3 27.5 80.4 84.7
Total reclassifications for the period, net of tax 9.8 (620.4) 31.0 (629.4)
Foreign currency translation adjustment        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total reclassifications for the period, net of tax 0.0 (0.3) 0.0 (0.3)
Net unrealized losses (gains) on investments        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total reclassifications for the period, net of tax 12.2 (618.5) 38.3 (623.7)
Net unrealized gains on derivative transactions        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total reclassifications for the period, net of tax (0.6) (0.6) (1.9) (1.8)
Unamortized net losses on Pension Plans        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total reclassifications for the period, net of tax (1.8) (1.0) (5.4) (3.6)
Reclassification out of Accumulated Other Comprehensive Income        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total reclassifications for the period, net of tax 9.8 (620.4) 31.0 (629.4)
Reclassification out of Accumulated Other Comprehensive Income | Foreign currency translation adjustment        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Underwriting, selling, general and administrative expenses 0.0 (0.8) 0.0 (0.8)
Provision for income taxes 0.0 0.5 0.0 0.5
Net of tax 0.0 (0.3) 0.0 (0.3)
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized losses (gains) on investments        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Provision for income taxes (3.3) 171.1 (10.2) 172.4
Net of tax 12.2 (618.5) 38.3 (623.7)
Net realized losses (gains) on investments 15.5 (789.6) 48.5 (796.1)
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized gains on derivative transactions        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Provision for income taxes 0.1 0.1 0.4 0.3
Net of tax (0.6) (0.6) (1.9) (1.8)
Interest expense (0.7) (0.7) (2.3) (2.1)
Reclassification out of Accumulated Other Comprehensive Income | Unamortized net losses on Pension Plans        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total before tax (2.3) (1.3) (6.9) (4.5)
Provision for income taxes 0.5 0.3 1.5 0.9
Total reclassifications for the period, net of tax (1.8) (1.0) (5.4) (3.6)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of net loss        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total before tax 1.1 1.8 3.3 5.4
Reclassification out of Accumulated Other Comprehensive Income | Amortization of prior service credit        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total before tax (3.4) (3.4) (10.2) (10.2)
Reclassification out of Accumulated Other Comprehensive Income | Settlement loss        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total before tax $ 0.0 $ 0.3 $ 0.0 $ 0.3