Segment Information Operating Results (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill, Accumulated Amortization		$ 15.0		$ 15.0
Segment Reporting, Asymmetrical Allocations to Segments		2.3
Revenues		$ 5,283.3	$ 4,741.5	10,178.6	$ 9,449.1
Pretax Profit (Loss)		$ 525.3	$ 437.4	$ 968.5	$ 919.2
Underwriting Operations
Segment Reporting Information [Line Items]
Combined Ratio		92.50%	92.60%	92.60%	93.00%
Revenues		$ 4,995.8	$ 4,513.5	$ 9,662.1	$ 8,915.8
Pretax Profit (Loss)		$ 373.8	$ 332.3	$ 715.4	$ 622.1
Underwriting Operations | Personal Lines
Segment Reporting Information [Line Items]
Combined Ratio		93.30%	93.60%	93.50%	93.70%
Revenues	[1]	$ 4,308.2	$ 4,066.3	$ 8,508.1	$ 8,031.7
Pretax Profit (Loss)	[1]	$ 290.0	$ 258.6	$ 552.7	$ 508.2
Underwriting Operations | Personal Lines | Agency Channel
Segment Reporting Information [Line Items]
Combined Ratio		92.40%	93.60%	91.90%	92.80%
Revenues		$ 2,276.4	$ 2,242.3	$ 4,520.9	$ 4,445.5
Pretax Profit (Loss)		$ 171.9	$ 142.9	$ 366.4	$ 321.5
Underwriting Operations | Personal Lines | Direct Channel
Segment Reporting Information [Line Items]
Combined Ratio		94.20%	93.70%	95.30%	94.80%
Revenues		$ 2,031.8	$ 1,824.0	$ 3,987.2	$ 3,586.2
Pretax Profit (Loss)		$ 118.1	$ 115.7	$ 186.3	$ 186.7
Underwriting Operations | Commercial Lines
Segment Reporting Information [Line Items]
Combined Ratio		83.00%	82.50%	83.10%	86.50%
Revenues		$ 489.3	$ 447.2	$ 955.7	$ 884.1
Pretax Profit (Loss)		$ 83.2	$ 78.4	$ 161.7	$ 119.3
Underwriting Operations | Property, Liability and Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Combined Ratio		99.50%	0.00%	99.50%	0.00%
Revenues		$ 198.7	$ 0.0	$ 198.7	$ 0.0
Pretax Profit (Loss)		1.0	0.0	1.0	0.0
Underwriting Operations | Other Indemnity
Segment Reporting Information [Line Items]
Revenues			0.0	(0.4)	0.0
Pretax Profit (Loss)		(0.4)	(4.7)	0.0	(5.4)
Fees and Other Revenues
Segment Reporting Information [Line Items]
Revenues	[2]	74.9	74.4	148.6	147.2
Service Businesses
Segment Reporting Information [Line Items]
Revenues		23.3	14.0	40.5	23.8
Pretax Profit (Loss)		2.8	1.1	4.1	1.2
Investments
Segment Reporting Information [Line Items]
Revenues	[3]	189.3	139.6	327.4	362.3
Pretax Profit (Loss)	[3]	183.6	133.6	316.4	352.2
Interest Expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)		$ (34.9)	$ (29.6)	$ (67.4)	$ (56.3)

[1] Personal auto insurance accounted for 92% of the total Personal Lines segment net premiums earned in both the second quarters and first six months of 2015 and 2014; 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 is 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.