Segment Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Segment Reporting Information [Line Items]
Revenues	$ 3,803	[1]	$ 3,772	[1]	$ 3,903
Income (loss) before income taxes	176		501		582
Commercial Lines
Segment Reporting Information [Line Items]
Revenues	2,200		2,156		2,201
Income (loss) before income taxes	(101)		15		(33)
Commercial Lines | Commercial Casualty Segment
Segment Reporting Information [Line Items]
Revenues	711		693		712
Commercial Lines | Commercial Property Segment
Segment Reporting Information [Line Items]
Revenues	497		489		485
Commercial Lines | Commercial Auto Segment
Segment Reporting Information [Line Items]
Revenues	394		384		394
Commercial Lines | Workers' Compensation
Segment Reporting Information [Line Items]
Revenues	318		311		326
Commercial Lines | Specialty packages
Segment Reporting Information [Line Items]
Revenues	138		149		147
Commercial Lines | Surety and executive risk
Segment Reporting Information [Line Items]
Revenues	103		95		104
Commercial Lines | Machinery and Equipment
Segment Reporting Information [Line Items]
Revenues	36		33		31
Commercial Lines | Operating Segments
Segment Reporting Information [Line Items]
Revenues	2,197		2,154		2,199
Commercial Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	3		2		2
Personal Lines
Segment Reporting Information [Line Items]
Revenues	763		723		686
Income (loss) before income taxes	(181)		(54)		(80)
Personal Lines | Personal Auto
Segment Reporting Information [Line Items]
Revenues	368		337		319
Personal Lines | Home Owner Segment
Segment Reporting Information [Line Items]
Revenues	294		289		276
Personal Lines | Other Segment
Segment Reporting Information [Line Items]
Revenues	100		95		90
Personal Lines | Operating Segments
Segment Reporting Information [Line Items]
Revenues	762		721		685
Personal Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	1		2		1
Excess And Surplus Lines Insurance
Segment Reporting Information [Line Items]
Revenues	70		49		27
Income (loss) before income taxes	6		(8)		(15)
Life Insurance Segment
Segment Reporting Information [Line Items]
Revenues	167		159		143
Income (loss) before income taxes	(3)		7		2
Investments Segment
Segment Reporting Information [Line Items]
Revenues	595		677		837
Income (loss) before income taxes	514		598		768
Other
Segment Reporting Information [Line Items]
Revenues	8		8		9
Income (loss) before income taxes	$ (59)		$ (57)		$ (60)

[1] Revenues including realized investment gains and losses, which are integral to our financial results over the long term, may cause this value to fluctuate substantially because we have substantial discretion in the timing of investment sales. Also, applicable accounting standards require us to recognize gains and losses from certain changes in fair values of securities and embedded derivatives without actual realization of those gains and losses. We discuss realized investment gains for the past three years in Item 7, Investments Results of Operations, Page 81.