Segment Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Revenues	$ 986	$ 929
Income (loss) before income taxes	112	75
Commercial Lines
Segment Reporting Information [Line Items]
Revenues	569	541
Income (loss) before income taxes	34	(22)
Commercial Lines | Commercial Casualty Segment
Segment Reporting Information [Line Items]
Revenues	181	172
Commercial Lines | Commercial Property Segment
Segment Reporting Information [Line Items]
Revenues	131	126
Commercial Lines | Commercial Auto Segment
Segment Reporting Information [Line Items]
Revenues	101	96
Commercial Lines | Workers' Compensation
Segment Reporting Information [Line Items]
Revenues	81	76
Commercial Lines | Specialty packages
Segment Reporting Information [Line Items]
Revenues	38	37
Commercial Lines | Surety and executive risk
Segment Reporting Information [Line Items]
Revenues	27	25
Commercial Lines | Machinery and Equipment
Segment Reporting Information [Line Items]
Revenues	9	8
Commercial Lines | Operating Segments
Segment Reporting Information [Line Items]
Revenues	568	540
Commercial Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	1	1
Personal Lines
Segment Reporting Information [Line Items]
Income (loss) before income taxes	(22)	(3)
Personal Lines | Personal Auto
Segment Reporting Information [Line Items]
Revenues	98	89
Personal Lines | Home Owner Segment
Segment Reporting Information [Line Items]
Revenues	84	76
Personal Lines | Other Segment
Segment Reporting Information [Line Items]
Revenues	27	25
Personal Lines | Operating Segments
Segment Reporting Information [Line Items]
Revenues	209	190
Excess And Surplus Lines Insurance
Segment Reporting Information [Line Items]
Revenues	21	15
Income (loss) before income taxes	(3)	(5)
Life Insurance Segment
Segment Reporting Information [Line Items]
Revenues	41	38
Income (loss) before income taxes	(3)	(3)
Investments Segment
Segment Reporting Information [Line Items]
Revenues	144	143
Income (loss) before income taxes	123	123
Other
Segment Reporting Information [Line Items]
Revenues	2	2
Income (loss) before income taxes	$ (17)	$ (15)