Segment Information Operating Results (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 9,530.5	$ 8,495.8	$ 28,281.2	$ 23,943.9
Pretax Profit (Loss)	1,010.3	1,130.5	3,821.4	2,921.4
Amortization of intangible assets	15.8	18.0	51.7	54.0
Personal Lines \| Agency \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	3,703.4	3,318.2	10,851.5	9,607.7
Pretax Profit (Loss)	380.2	372.2	1,267.9	1,142.2
Personal Lines \| Direct \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	3,804.0	3,337.2	11,113.7	9,565.3
Pretax Profit (Loss)	291.7	294.4	940.2	880.3
Operating Segments \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	9,012.2	7,930.5	26,296.7	22,738.7
Pretax Profit (Loss)	732.5	771.4	2,533.0	2,299.2
Operating Segments \| Personal Lines \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	7,507.4	6,655.4	21,965.2	19,173.0
Pretax Profit (Loss)	$ 671.9	$ 666.6	$ 2,208.1	$ 2,022.5
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	$ 1,106.9	$ 939.6	$ 3,190.4	$ 2,632.5
Pretax Profit (Loss)	69.7	112.7	360.7	307.8
Operating Segments \| Property \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	397.9	335.5	1,141.1	933.2
Pretax Profit (Loss)	(9.1)	(7.8)	(35.8)	(31.2)
Amortization of intangible assets	15.8	18.0	51.7	54.0
Operating Segments \| Other Indemnity \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	0.0	0.0	0.0	0.0
Pretax Profit (Loss)	0.0	(0.1)	0.0	0.1
Segment Reconciling Items \| Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	138.4	122.6	403.4	342.4
Segment Reconciling Items \| Service businesses
Segment Reporting Information [Line Items]
Revenues	51.5	42.5	144.1	119.6
Pretax Profit (Loss)	2.8	6.7	12.0	17.5
Segment Reconciling Items \| Investment income
Segment Reporting Information [Line Items]
Revenues	328.4	400.2	1,437.0	743.2
Pretax Profit (Loss)	322.5	394.4	1,418.7	725.2
Segment Reconciling Items \| Interest expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	$ (47.5)	$ (42.0)	$ (142.3)	$ (120.5)

Percentage of segment revenue attributable to personal auto insurance.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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