Segment Information - Operating Results (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Revenues	$ 19,719.0	$ 15,560.6	$ 55,095.8	$ 45,217.3
Pretax Profit (Loss)	2,944.8	1,410.2	7,745.1	2,400.3
Personal Lines | Agency | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	6,627.8	5,414.0	18,698.9	15,481.4
Pretax Profit (Loss)	870.1	335.7	2,609.3	427.1
Personal Lines | Direct | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	8,180.2	6,361.8	22,796.2	18,259.9
Pretax Profit (Loss)	664.5	706.2	2,490.8	810.6
Operating Segments | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	18,296.7	14,894.3	51,654.8	42,891.8
Pretax Profit (Loss)	2,004.4	1,136.2	5,640.9	1,213.5
Operating Segments | Personal Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	14,808.0	11,775.8	41,495.1	33,741.3
Pretax Profit (Loss)	$ 1,534.6	$ 1,041.9	$ 5,100.1	$ 1,237.7
Personal auto insurance percentage of the total personal lines segment net premiums earned	95.00%	94.00%	95.00%	94.00%
Operating Segments | Commercial Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	$ 2,726.7	$ 2,486.6	$ 7,948.6	$ 7,296.8
Pretax Profit (Loss)	308.4	23.0	821.2	147.4
Operating Segments | Property | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	761.8	631.8	2,210.5	1,852.8
Pretax Profit (Loss)	163.7	74.6	(277.2)	(164.9)
Operating Segments | Other Indemnity | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	0.2	0.1	0.6	0.9
Pretax Profit (Loss)	(2.3)	(3.3)	(3.2)	(6.7)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Interest expense	(69.9)	(69.7)	(209.1)	(198.7)
Segment Reconciling Items | Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	278.1	223.7	774.4	656.6
Segment Reconciling Items | Service businesses
Segment Reporting Information [Line Items]
Revenues	117.3	81.4	307.8	234.9
Pretax Profit (Loss)	(9.4)	(10.3)	(25.3)	(29.7)
Segment Reconciling Items | Investments
Segment Reporting Information [Line Items]
Revenues	1,026.9	361.2	2,358.8	1,434.0
Pretax Profit (Loss)	$ 1,019.7	$ 354.0	$ 2,338.6	$ 1,415.2