Segment Information (Detail) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Revenues	$ 944	$ 1,071	$ 2,848	$ 2,836
Income (loss) before income taxes	11	221	(10)	327
Commercial Lines
Segment Reporting Information [Line Items]
Revenues	558	548	1,632	1,610
Income (loss) before income taxes	(44)	(18)	(193)	(37)
Commercial Lines | Commercial Casualty Segment
Segment Reporting Information [Line Items]
Revenues	180	182	532	518
Commercial Lines | Commercial Property Segment
Segment Reporting Information [Line Items]
Revenues	128	123	369	365
Commercial Lines | Commercial Auto Segment
Segment Reporting Information [Line Items]
Revenues	100	96	292	287
Commercial Lines | Workers' Compensation
Segment Reporting Information [Line Items]
Revenues	78	77	235	230
Commercial Lines | Specialty
Segment Reporting Information [Line Items]
Revenues	36	38	100	112
Commercial Lines | Surety Segment
Segment Reporting Information [Line Items]
Revenues	26	22	76	71
Commercial Lines | Machinery and Equipment
Segment Reporting Information [Line Items]
Revenues	9	9	26	25
Commercial Lines | Operating Segments
Segment Reporting Information [Line Items]
Revenues	557	547	1,630	1,608
Commercial Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	1	1	2	2
Personal Lines
Segment Reporting Information [Line Items]
Revenues	193	182	564	536
Income (loss) before income taxes	(37)	(7)	(182)	(52)
Personal Lines | Operating Segments
Segment Reporting Information [Line Items]
Revenues	193	182	563	535
Personal Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues			1	1
Personal Lines | Personal Auto
Segment Reporting Information [Line Items]
Revenues	94	86	273	250
Personal Lines | Home Owner Segment
Segment Reporting Information [Line Items]
Revenues	74	72	216	214
Personal Lines | Other Segment
Segment Reporting Information [Line Items]
Revenues	25	24	74	71
Excess And Surplus Lines Insurance
Segment Reporting Information [Line Items]
Revenues	19	14	51	36
Income (loss) before income taxes	1	(3)	1	(11)
Life Insurance Segment
Segment Reporting Information [Line Items]
Revenues	44	41	125	121
Income (loss) before income taxes	(3)	(1)	(1)	1
Investments Segment
Segment Reporting Information [Line Items]
Revenues	128	283	470	528
Income (loss) before income taxes	107	262	409	468
Other
Segment Reporting Information [Line Items]
Revenues	2	3	6	5
Income (loss) before income taxes	$ (13)	$ (12)	$ (44)	$ (42)