Segment Information - Operating Results (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Revenues	$ 18,134.3	$ 15,353.5	$ 35,376.8	$ 29,656.7
Pretax Profit (Loss)	1,860.4	435.7	4,800.3	990.1
Personal Lines | Agency | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	6,213.4	5,207.2	12,071.1	10,067.4
Pretax Profit (Loss)	788.5	(71.2)	1,739.2	91.4
Personal Lines | Direct | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	7,595.5	6,180.7	14,616.0	11,898.1
Pretax Profit (Loss)	782.7	126.5	1,826.3	104.4
Operating Segments | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	17,209.5	14,464.4	33,358.1	27,997.5
Pretax Profit (Loss)	1,386.6	(64.3)	3,636.5	77.3
Operating Segments | Personal Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	13,808.9	11,387.9	26,687.1	21,965.5
Pretax Profit (Loss)	$ 1,571.2	$ 55.3	$ 3,565.5	$ 195.8
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	$ 2,664.5	$ 2,454.1	$ 5,221.9	$ 4,810.2
Pretax Profit (Loss)	303.8	87.2	512.8	124.4
Operating Segments | Property | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	735.9	622.3	1,448.7	1,221.0
Pretax Profit (Loss)	(487.8)	(206.8)	(440.9)	(239.5)
Operating Segments | Other Indemnity | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	0.2	0.1	0.4	0.8
Pretax Profit (Loss)	(0.6)	0.0	(0.9)	(3.4)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Interest expense	(69.6)	(65.7)	(139.2)	(129.0)
Segment Reconciling Items | Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	259.8	226.7	496.3	432.9
Segment Reconciling Items | Service businesses
Segment Reporting Information [Line Items]
Revenues	106.3	81.0	190.5	153.5
Pretax Profit (Loss)	(8.0)	(9.6)	(15.9)	(19.4)
Segment Reconciling Items | Investments
Segment Reporting Information [Line Items]
Revenues	558.7	581.4	1,331.9	1,072.8
Pretax Profit (Loss)	$ 551.4	$ 575.3	$ 1,318.9	$ 1,061.2