Segment Information - Operating Results (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Revenues	$ 11,519.2	$ 11,905.5	$ 23,360.7	$ 23,350.7
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(635.9)	999.2	(245.7)	2,867.1
Amortization of intangible assets	5.5	14.3	20.1	28.5
Personal Lines | Agency | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	4,366.5	4,220.3	8,689.8	8,318.5
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	260.3	207.5	548.9	755.0
Personal Lines | Direct | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	4,905.9	4,633.9	9,699.5	9,065.6
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	198.4	128.4	348.8	543.0
Operating Segments | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	12,147.9	10,982.3	23,950.8	21,402.5
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	538.5	382.8	1,187.3	1,502.7
Operating Segments | Personal Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	9,272.4	8,854.2	18,389.3	17,384.1
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	$ 458.7	$ 335.9	$ 897.7	$ 1,298.0
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	$ 2,304.4	$ 1,621.8	$ 4,431.6	$ 3,039.6
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	243.0	130.1	445.4	358.6
Operating Segments | Property
Segment Reporting Information [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(381.7)		(373.4)
Operating Segments | Property | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	570.5	502.3	1,128.6	974.8
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(156.9)	(83.3)	(148.6)	(154.0)
Amortization of intangible assets	5.0	14.1	19.1	28.3
Operating Segments | Property | Property - Goodwill impairment
Segment Reporting Information [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(224.8)	0.0	(224.8)	0.0
Operating Segments | Other Indemnity | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	0.6	4.0	1.3	4.0
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(6.3)	0.1	(7.2)	0.1
Segment Reconciling Items | Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	176.5	176.2	350.5	341.9
Segment Reconciling Items | Service businesses
Segment Reporting Information [Line Items]
Revenues	80.1	74.5	147.8	128.3
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	4.6	6.6	9.1	11.1
Segment Reconciling Items | Investment income
Segment Reporting Information [Line Items]
Revenues	(885.3)	672.5	(1,088.4)	1,478.0
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	(891.2)	666.2	(1,100.0)	1,466.1
Segment Reconciling Items | Interest expense
Segment Reporting Information [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	$ (63.0)	$ (56.4)	$ (117.3)	$ (112.8)