Segment Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenues	$ 1,103	$ 986
Income (loss) before income taxes	217	112
Identifiable assets	17,037		16,548
Property Casualty Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	2,403		2,395
Commercial Lines
Segment Reporting Information [Line Items]
Revenues	631	569
Income (loss) before income taxes	58	34
Commercial Lines | Commercial Casualty Segment
Segment Reporting Information [Line Items]
Revenues	204	181
Commercial Lines | Commercial Property Segment
Segment Reporting Information [Line Items]
Revenues	147	131
Commercial Lines | Commercial Auto Segment
Segment Reporting Information [Line Items]
Revenues	114	101
Commercial Lines | Workers' Compensation
Segment Reporting Information [Line Items]
Revenues	88	81
Commercial Lines | Specialty Packages
Segment Reporting Information [Line Items]
Revenues	39	38
Commercial Lines | Surety and Executive Risk
Segment Reporting Information [Line Items]
Revenues	29	27
Commercial Lines | Machinery and Equipment
Segment Reporting Information [Line Items]
Revenues	10	9
Commercial Lines | Commercial Lines Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	631	568
Commercial Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	0	1
Personal Lines
Segment Reporting Information [Line Items]
Income (loss) before income taxes	20	(22)
Personal Lines | Personal Auto
Segment Reporting Information [Line Items]
Revenues	107	98
Personal Lines | Home Owner Segment
Segment Reporting Information [Line Items]
Revenues	96	84
Personal Lines | Other Personal Lines
Segment Reporting Information [Line Items]
Revenues	28	27
Personal Lines | Commercial Lines Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	231	209
Excess And Surplus Lines Insurance
Segment Reporting Information [Line Items]
Revenues	27	21
Income (loss) before income taxes	0	(3)
Life Insurance Segment
Segment Reporting Information [Line Items]
Revenues	43	41
Income (loss) before income taxes	7	(3)
Identifiable assets	1,193		1,201
Life Insurance Segment | Life Insurance Excluding Account Investment Fees
Segment Reporting Information [Line Items]
Revenues	42	41
Life Insurance Segment | Seperate Account Investment Management Fees
Segment Reporting Information [Line Items]
Revenues	1	0
Investments Segment
Segment Reporting Information [Line Items]
Revenues	169	144
Income (loss) before income taxes	148	123
Identifiable assets	13,099		12,599
Investments Segment | Investment Income, Net
Segment Reporting Information [Line Items]
Revenues	128	131
Investments Segment | Realized Investment Gains, Net
Segment Reporting Information [Line Items]
Revenues	41	13
Other
Segment Reporting Information [Line Items]
Revenues	2	2
Income (loss) before income taxes	(16)	(17)
Identifiable assets	$ 342		$ 353