Segment Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenues	$ 1,104	$ 1,020	$ 2,207	$ 2,006
Net Investment Income	131	132	259	263
Realized gains on investments	14	6	55	19
Income (loss) before income taxes	148	29	365	141
Identifiable assets	17,034		17,034		16,548
Property Casualty Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	2,489		2,489		2,395
Commercial Lines
Segment Reporting Information [Line Items]
Revenues	646	591	1,277	1,160
Income (loss) before income taxes	34	(20)	92	14
Commercial Lines | Commercial Casualty Segment
Segment Reporting Information [Line Items]
Revenues	211	191	415	372
Commercial Lines | Commercial Property Segment
Segment Reporting Information [Line Items]
Revenues	152	134	299	265
Commercial Lines | Commercial Auto Segment
Segment Reporting Information [Line Items]
Revenues	117	106	231	207
Commercial Lines | Workers' Compensation
Segment Reporting Information [Line Items]
Revenues	87	85	175	166
Commercial Lines | Specialty Packages
Segment Reporting Information [Line Items]
Revenues	37	37	76	75
Commercial Lines | Surety and Executive Risk
Segment Reporting Information [Line Items]
Revenues	30	27	59	54
Commercial Lines | Machinery and Equipment
Segment Reporting Information [Line Items]
Revenues	11	10	21	19
Commercial Lines | Commercial Lines Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	645	590	1,276	1,158
Commercial Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	1	1	1	2
Personal Lines
Segment Reporting Information [Line Items]
Revenues	238	215	469	424
Income (loss) before income taxes	(1)	(55)	19	(77)
Personal Lines | Personal Auto
Segment Reporting Information [Line Items]
Revenues	109	100	216	198
Personal Lines | Home Owner Segment
Segment Reporting Information [Line Items]
Revenues	99	87	195	171
Personal Lines | Other Personal Lines
Segment Reporting Information [Line Items]
Revenues	29	27	57	54
Personal Lines | Commercial Lines Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	237	214	468	423
Personal Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	1	1	1	1
Excess And Surplus Lines Insurance
Segment Reporting Information [Line Items]
Revenues	28	22	55	43
Income (loss) before income taxes	1	(2)	1	(5)
Life Insurance Segment
Segment Reporting Information [Line Items]
Revenues	45	51	88	92
Income (loss) before income taxes	3	2	10	(1)
Identifiable assets	1,254		1,254		1,201
Life Insurance Segment | Life Insurance Excluding Account Investment Fees
Segment Reporting Information [Line Items]
Revenues	44	51	86	92
Life Insurance Segment | Seperate Account Investment Management Fees
Segment Reporting Information [Line Items]
Revenues	1	0	2	0
Investments Segment
Segment Reporting Information [Line Items]
Revenues	145	138	314	282
Income (loss) before income taxes	127	118	275	241
Identifiable assets	12,983		12,983		12,599
Investments Segment | Investment Income, Net
Segment Reporting Information [Line Items]
Revenues	131	132
Net Investment Income				263
Investments Segment | Realized Investment Gains, Net
Segment Reporting Information [Line Items]
Revenues	14	6		19
Other
Segment Reporting Information [Line Items]
Revenues	2	3	4	5
Income (loss) before income taxes	(16)	(14)	(32)	(31)
Identifiable assets	$ 308		$ 308		$ 353