Segment Information Operating Results (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Segment Reporting Information [Line Items]
Revenues	$ 4,741.5		$ 4,593.6		$ 9,449.1		$ 9,030.8
Pretax Profit (Loss)	437.4		486.3		919.2		949.4
Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	4,513.5		4,277.0		8,915.8		8,456.3
Pretax Profit (Loss)	332.3		285.8		622.1		603.5
Underwriting Operations | Personal Lines
Segment Reporting Information [Line Items]
Revenues	4,066.3	[1]	3,836.9	[1]	8,031.7	[1]	7,585.7	[1]
Pretax Profit (Loss)	258.6	[1]	257.5	[1]	508.2	[1]	559.6	[1]
Underwriting Operations | Personal Lines | Agency Channel
Segment Reporting Information [Line Items]
Revenues	2,242.3		2,151.5		4,445.5		4,258.7
Pretax Profit (Loss)	142.9		131.5		321.5		306.4
Underwriting Operations | Personal Lines | Direct Channel
Segment Reporting Information [Line Items]
Revenues	1,824.0		1,685.4		3,586.2		3,327.0
Pretax Profit (Loss)	115.7		126.0		186.7		253.2
Underwriting Operations | Commercial Lines
Segment Reporting Information [Line Items]
Revenues	447.2		440.1		884.1		870.5
Pretax Profit (Loss)	78.4		32.1		119.3		47.4
Underwriting Operations | Other Indemnity
Segment Reporting Information [Line Items]
Revenues	0		0		0		0.1
Pretax Profit (Loss)	(4.7)		(3.8)		(5.4)		(3.5)
Fees and Other Revenues
Segment Reporting Information [Line Items]
Revenues	74.4	[2]	70.9	[2]	147.2	[2]	139.3	[2]
Service Businesses
Segment Reporting Information [Line Items]
Revenues	14.0		10.6		23.8		19.0
Pretax Profit (Loss)	1.1		0.2		1.2		0
Investments
Segment Reporting Information [Line Items]
Revenues	139.6	[3]	235.1	[3]	362.3	[3]	416.2	[3]
Pretax Profit (Loss)	133.6	[3]	230.7	[3]	352.2	[3]	406.9	[3]
Interest Expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	$ (29.6)		$ (30.4)		$ (56.3)		$ (61.0)

[1] Personal auto insurance accounted for 92% of the total Personal Lines segment net premiums earned in both the second quarter and first six months of 2014, compared to 91% for the same periods last year; 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.