Segment Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Revenues	$ 1,020	$ 975	$ 2,006	$ 1,904
Income (loss) before income taxes	29	(99)	141	(24)
Commercial Lines
Segment Reporting Information [Line Items]
Revenues	591	533	1,160	1,074
Income (loss) before income taxes	(20)	(129)	14	(151)
Commercial Lines | Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	590	533	1,158	1,073
Commercial Lines | Insurance Premiums | Commercial Casualty Segment
Segment Reporting Information [Line Items]
Revenues	191	180	372	352
Commercial Lines | Insurance Premiums | Commercial Property Segment
Segment Reporting Information [Line Items]
Revenues	134	115	265	241
Commercial Lines | Insurance Premiums | Commercial Auto Segment
Segment Reporting Information [Line Items]
Revenues	106	96	207	192
Commercial Lines | Insurance Premiums | Workers' Compensation
Segment Reporting Information [Line Items]
Revenues	85	81	166	157
Commercial Lines | Insurance Premiums | Specialty packages
Segment Reporting Information [Line Items]
Revenues	37	27	75	64
Commercial Lines | Insurance Premiums | Surety and executive risk
Segment Reporting Information [Line Items]
Revenues	27	25	54	50
Commercial Lines | Insurance Premiums | Machinery and Equipment
Segment Reporting Information [Line Items]
Revenues	10	9	19	17
Commercial Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	1		2	1
Personal Lines
Segment Reporting Information [Line Items]
Revenues	215	181	424	371
Income (loss) before income taxes	(55)	(142)	(77)	(145)
Personal Lines | Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	214	180	423	370
Personal Lines | Insurance Premiums | Personal Auto
Segment Reporting Information [Line Items]
Revenues	100	90	198	179
Personal Lines | Insurance Premiums | Home Owner Segment
Segment Reporting Information [Line Items]
Revenues	87	66	171	142
Personal Lines | Insurance Premiums | Other
Segment Reporting Information [Line Items]
Revenues	27	24	54	49
Personal Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	1	1	1	1
Excess And Surplus Lines Insurance
Segment Reporting Information [Line Items]
Revenues	22	17	43	32
Income (loss) before income taxes	(2)	4	(5)	(1)
Life Insurance Segment
Segment Reporting Information [Line Items]
Revenues	51	43	92	81
Income (loss) before income taxes	2	5	(1)	2
Investments Segment
Segment Reporting Information [Line Items]
Revenues	138	199	282	342
Income (loss) before income taxes	118	179	241	302
Other Segment
Segment Reporting Information [Line Items]
Revenues	3	2	5	4
Income (loss) before income taxes	$ (14)	$ (16)	$ (31)	$ (31)