Segment Information Operating Results (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Revenues	$ 6,605.7	$ 5,819.3	$ 12,927.4	$ 11,376.8
Pretax Profit (Loss)	554.6	287.3	1,196.1	674.7
Amortization of intangible assets	15.5	15.6	31.0	31.1
Personal Lines \| Agency \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	2,752.5	2,423.3	5,384.0	4,771.2
Pretax Profit (Loss)	205.5	84.5	455.2	255.9
Personal Lines \| Direct \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	2,650.0	2,334.8	5,173.7	4,555.5
Pretax Profit (Loss)	162.7	73.6	338.0	153.1
Operating Segments \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	6,313.3	5,561.8	12,340.0	10,879.2
Pretax Profit (Loss)	428.0	175.6	928.0	462.4
Operating Segments \| Personal Lines \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	5,402.5	4,758.1	10,557.7	9,326.7
Pretax Profit (Loss)	$ 368.2	$ 158.1	$ 793.2	$ 409.0
Personal auto insurance percentage of the total personal lines segment net premiums earned	93.00%	92.00%	93.00%	92.00%
Operating Segments \| Commercial Lines \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	$ 671.7	$ 593.4	$ 1,317.2	$ 1,142.2
Pretax Profit (Loss)	56.2	31.2	123.6	92.3
Operating Segments \| Property \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	239.1	210.3	465.1	410.3
Pretax Profit (Loss)	3.7	(13.6)	11.5	(38.1)
Amortization of intangible assets	15.5	15.6	31.0	31.1
Operating Segments \| Other Indemnity \| Underwriting operations
Segment Reporting Information [Line Items]
Revenues	0.0	0.0	0.0	0.0
Pretax Profit (Loss)	(0.1)	(0.1)	(0.3)	(0.8)
Segment Reconciling Items \| Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	88.8	82.5	174.0	161.4
Segment Reconciling Items \| Service businesses
Segment Reporting Information [Line Items]
Revenues	32.7	26.5	61.2	51.5
Pretax Profit (Loss)	5.7	2.8	8.3	6.2
Segment Reconciling Items \| Investment income
Segment Reporting Information [Line Items]
Revenues	170.9	146.9	352.0	283.1
Pretax Profit (Loss)	164.3	141.6	339.8	273.0
Segment Reconciling Items \| Gains on extinguishment of debt
Segment Reporting Information [Line Items]
Revenues	0.0	1.6	0.2	1.6
Pretax Profit (Loss)	0.0	1.6	0.2	1.6
Segment Reconciling Items \| Interest expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	$ (43.4)	$ (34.3)	$ (80.2)	$ (68.5)

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
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Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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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