Operating Results (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Segment Reporting Information [Line Items]
Revenues	$ 4,521.3		$ 4,423.9		$ 13,552.1		$ 12,733.3
Pretax Profit (Loss)	345.0		412.4		1,294.4		964.2
Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	4,303.0		4,054.8		12,759.3		11,912.4
Pretax Profit (Loss)	249.4		165.2		852.9		489.0
Underwriting Operations | Personal Lines
Segment Reporting Information [Line Items]
Revenues	3,856.1	[1]	3,630.4	[1]	11,441.8	[1]	10,692.4	[1]
Pretax Profit (Loss)	229.3	[1]	158.2	[1]	788.9	[1]	431.1	[1]
Underwriting Operations | Personal Lines | Agency Channel
Segment Reporting Information [Line Items]
Revenues	2,162.7		2,047.7		6,421.4		6,032.3
Pretax Profit (Loss)	122.0		52.3		428.4		237.9
Underwriting Operations | Personal Lines | Direct Channel
Segment Reporting Information [Line Items]
Revenues	1,693.4		1,582.7		5,020.4		4,660.1
Pretax Profit (Loss)	107.3		105.9		360.5		193.2
Underwriting Operations | Commercial Auto
Segment Reporting Information [Line Items]
Revenues	446.8		424.3		1,317.3		1,219.2
Pretax Profit (Loss)	26.3		7.5		73.7		61.0
Underwriting Operations | Other Indemnity
Segment Reporting Information [Line Items]
Revenues	0.1		0.1		0.2		0.8
Pretax Profit (Loss)	(6.2)		(0.5)		(9.7)		(3.1)
Fees and Other Revenues
Segment Reporting Information [Line Items]
Revenues	76.0	[2]	78.4	[2]	215.3	[2]	213.5	[2]
Service Businesses
Segment Reporting Information [Line Items]
Revenues	11.3		9.9		30.3		28.3
Pretax Profit (Loss)	0.2		0.3		0.2		0.6
Investments
Segment Reporting Information [Line Items]
Revenues	135.3	[3]	280.9	[3]	551.5	[3]	580.9	[3]
Pretax Profit (Loss)	130.1	[3]	277.6	[3]	537.0	[3]	569.6	[3]
Gains (losses) on extinguishment of debt
Segment Reporting Information [Line Items]
Revenues	(4.3)		(0.1)		(4.3)		(1.8)
Pretax Profit (Loss)	(4.3)		(0.1)		(4.3)		(1.8)
Interest Expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	$ (30.4)		$ (30.6)		$ (91.4)		$ (93.2)

[1] Personal auto insurance accounted for 91% of the total Personal Lines segment net premiums earned in all periods; insurance for our special lines products (e.g., motorcycles, ATVs, RVs, mobile homes, watercraft, and snowmobiles) accounted for the balance of the Personal Lines net premiums earned.

[2] Pretax profit (loss) for fees and other revenues are allocated to operating segments.

[3] Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment expenses.