Segment Information - Operating Results (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Revenues	$ 17,242.5	$ 14,303.2
Pretax Profit (Loss)	2,939.9	554.4
Personal Lines | Agency | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	5,857.7	4,860.2
Pretax Profit (Loss)	950.7	162.6
Personal Lines | Direct | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	7,020.5	5,717.4
Pretax Profit (Loss)	1,043.6	(22.1)
Operating Segments | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	16,148.6	13,533.1
Pretax Profit (Loss)	2,249.9	141.6
Operating Segments | Personal Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	12,878.2	10,577.6
Pretax Profit (Loss)	$ 1,994.3	$ 140.5
Personal auto insurance percentage of the total personal lines segment net premiums earned	95.00%	94.00%
Operating Segments | Commercial Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	$ 2,557.4	$ 2,356.1
Pretax Profit (Loss)	209.0	37.2
Operating Segments | Property | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	712.8	598.7
Pretax Profit (Loss)	46.9	(32.7)
Operating Segments | Other Indemnity | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	0.2	0.7
Pretax Profit (Loss)	(0.3)	(3.4)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Interest expense	(69.6)	(63.3)
Segment Reconciling Items | Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	236.5	206.2
Segment Reconciling Items | Service businesses
Segment Reporting Information [Line Items]
Revenues	84.2	72.5
Pretax Profit (Loss)	(7.9)	(9.8)
Segment Reconciling Items | Investments
Segment Reporting Information [Line Items]
Revenues	773.2	491.4
Pretax Profit (Loss)	$ 767.5	$ 485.9