Segment Information (Detail) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Segment Reporting Information [Line Items]
Revenues	$ 975	$ 878	$ 1,904	$ 1,765
Income (loss) before income taxes	(97)	21	(21)	106
Commercial Lines
Segment Reporting Information [Line Items]
Revenues	533	538	1,074	1,062
Income (loss) before income taxes	(128)	(10)	(149)	(19)
Commercial Lines | Commercial Casualty Segment
Segment Reporting Information [Line Items]
Revenues	180	172	352	336
Commercial Lines | Commercial Property Segment
Segment Reporting Information [Line Items]
Revenues	115	121	241	242
Commercial Lines | Commercial Auto Segment
Segment Reporting Information [Line Items]
Revenues	96	96	192	191
Commercial Lines | Workers' Compensation
Segment Reporting Information [Line Items]
Revenues	81	79	157	153
Commercial Lines | Specialty
Segment Reporting Information [Line Items]
Revenues	27	37	64	74
Commercial Lines | Surety Segment
Segment Reporting Information [Line Items]
Revenues	25	25	50	49
Commercial Lines | Machinery and Equipment
Segment Reporting Information [Line Items]
Revenues	9	8	17	16
Commercial Lines | Operating Segments
Segment Reporting Information [Line Items]
Revenues	533	538	1,073	1,061
Commercial Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues			1	1
Personal Lines
Segment Reporting Information [Line Items]
Revenues	181	180	371	354
Income (loss) before income taxes	(142)	(40)	(145)	(45)
Personal Lines | Operating Segments
Segment Reporting Information [Line Items]
Revenues	180	179	370	353
Personal Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	1	1	1	1
Personal Lines | Personal Auto
Segment Reporting Information [Line Items]
Revenues	90	83	179	164
Personal Lines | Home Owner Segment
Segment Reporting Information [Line Items]
Revenues	66	72	142	142
Personal Lines | Other Segment
Segment Reporting Information [Line Items]
Revenues	24	24	49	47
Excess And Surplus Lines Insurance
Segment Reporting Information [Line Items]
Revenues	17	11	32	22
Income (loss) before income taxes	5	(4)		(8)
Life Insurance Segment
Segment Reporting Information [Line Items]
Revenues	43	41	81	80
Income (loss) before income taxes	5	2	2	2
Investments Segment
Segment Reporting Information [Line Items]
Revenues	199	107	342	245
Income (loss) before income taxes	179	87	302	206
Other
Segment Reporting Information [Line Items]
Revenues	2	1	4	2
Income (loss) before income taxes	$ (16)	$ (14)	$ (31)	$ (30)