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Segment Information - Operating Results (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Segment Reporting Information [Line Items]        
Revenues $ 15,560.6 $ 12,780.2 $ 45,217.3 $ 36,140.9
Pretax Profit (Loss) 1,410.2 142.5 2,400.3 (103.2)
Amortization of intangible assets 2.9 5.4 11.4 25.5
Personal Lines | Agency | Underwriting operations        
Segment Reporting Information [Line Items]        
Revenues 5,414.0 4,441.9 15,481.4 13,131.7
Pretax Profit (Loss) 335.7 (41.3) 427.1 507.6
Personal Lines | Direct | Underwriting operations        
Segment Reporting Information [Line Items]        
Revenues 6,361.8 5,077.4 18,259.9 14,776.9
Pretax Profit (Loss) 706.2 40.6 810.6 389.4
Operating Segments | Underwriting operations        
Segment Reporting Information [Line Items]        
Revenues 14,894.3 12,398.9 42,891.8 36,349.7
Pretax Profit (Loss) 1,136.2 94.3 1,213.5 1,281.6
Operating Segments | Personal Lines | Underwriting operations        
Segment Reporting Information [Line Items]        
Revenues 11,775.8 9,519.3 33,741.3 27,908.6
Pretax Profit (Loss) $ 1,041.9 $ (0.7) $ 1,237.7 $ 897.0
Personal auto insurance percentage of the total personal lines segment net premiums earned 94.00% 94.00% 94.00% 94.00%
Operating Segments | Commercial Lines | Underwriting operations        
Segment Reporting Information [Line Items]        
Revenues $ 2,486.6 $ 2,317.9 $ 7,296.8 $ 6,749.5
Pretax Profit (Loss) 23.0 238.1 147.4 683.5
Operating Segments | Property        
Segment Reporting Information [Line Items]        
Pretax Profit (Loss)       514.4
Operating Segments | Property | Underwriting operations        
Segment Reporting Information [Line Items]        
Revenues 631.8 561.0 1,852.8 1,689.6
Pretax Profit (Loss) 74.6 (141.0) (164.9) (289.6)
Amortization of intangible assets 2.9 5.0 10.7 24.1
Operating Segments | Property | Property - Goodwill impairment        
Segment Reporting Information [Line Items]        
Pretax Profit (Loss) 0.0 0.0 0.0 (224.8)
Operating Segments | Other Indemnity | Underwriting operations        
Segment Reporting Information [Line Items]        
Revenues 0.1 0.7 0.9 2.0
Pretax Profit (Loss) (3.3) (2.1) (6.7) (9.3)
Segment Reconciling Items | Fees and other revenues        
Segment Reporting Information [Line Items]        
Revenues 223.7 181.4 656.6 531.9
Segment Reconciling Items | Service businesses        
Segment Reporting Information [Line Items]        
Revenues 81.4 82.7 234.9 230.5
Pretax Profit (Loss) (10.3) (0.1) (29.7) 9.0
Segment Reconciling Items | Investment income        
Segment Reporting Information [Line Items]        
Revenues 361.2 117.2 1,434.0 (971.2)
Pretax Profit (Loss) 354.0 111.4 1,415.2 (988.6)
Segment Reconciling Items | Interest expense        
Segment Reporting Information [Line Items]        
Pretax Profit (Loss) $ (69.7) $ (63.1) $ (198.7) $ (180.4)