Segment Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Revenues	$ 1,035	$ 944	$ 3,041	$ 2,848
Income (loss) before income taxes	152	11	293	(13)
Commercial Lines
Segment Reporting Information [Line Items]
Revenues	607	558	1,767	1,632
Income (loss) before income taxes	60	(44)	74	(195)
Commercial Lines | Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	607	557	1,765	1,630
Commercial Lines | Insurance Premiums | Commercial Casualty Segment
Segment Reporting Information [Line Items]
Revenues	197	180	569	532
Commercial Lines | Insurance Premiums | Commercial Property Segment
Segment Reporting Information [Line Items]
Revenues	138	128	403	369
Commercial Lines | Insurance Premiums | Commercial Auto Segment
Segment Reporting Information [Line Items]
Revenues	108	100	315	292
Commercial Lines | Insurance Premiums | Workers' Compensation
Segment Reporting Information [Line Items]
Revenues	89	78	255	235
Commercial Lines | Insurance Premiums | Specialty packages
Segment Reporting Information [Line Items]
Revenues	37	36	112	100
Commercial Lines | Insurance Premiums | Surety and executive risk
Segment Reporting Information [Line Items]
Revenues	28	26	82	76
Commercial Lines | Insurance Premiums | Machinery and Equipment
Segment Reporting Information [Line Items]
Revenues	10	9	29	26
Commercial Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues		1	2	2
Personal Lines
Segment Reporting Information [Line Items]
Revenues	220	193	644	564
Income (loss) before income taxes	(12)	(38)	(89)	(183)
Personal Lines | Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	219	193	642	563
Personal Lines | Insurance Premiums | Personal Auto
Segment Reporting Information [Line Items]
Revenues	101	94	299	273
Personal Lines | Insurance Premiums | Home Owner Segment
Segment Reporting Information [Line Items]
Revenues	90	74	261	216
Personal Lines | Insurance Premiums | Other
Segment Reporting Information [Line Items]
Revenues	28	25	82	74
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	25	19	68	51
Income (loss) before income taxes	(3)	2	(8)	1
Life Insurance Segment
Segment Reporting Information [Line Items]
Revenues	39	44	131	125
Income (loss) before income taxes		(3)	(2)	(1)
Investments Segment
Segment Reporting Information [Line Items]
Revenues	142	128	424	470
Income (loss) before income taxes	121	107	362	409
Other Segment
Segment Reporting Information [Line Items]
Revenues	2	2	7	6
Income (loss) before income taxes	$ (14)	$ (13)	$ (44)	$ (44)