Segment Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenues	$ 1,152	$ 1,035	$ 3,359	$ 3,041
Net Investment Income	133	132	392	395
Realized gains on investments	22	10	77	29
Income (loss) before income taxes	182	152	547	293
Identifiable assets	17,339		17,339		16,548
Property Casualty Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	2,479		2,479		2,395
Commercial Lines
Segment Reporting Information [Line Items]
Revenues	681	607	1,958	1,767
Income (loss) before income taxes	43	60	135	74
Commercial Lines | Commercial Casualty Segment
Segment Reporting Information [Line Items]
Revenues	221	197	636	569
Commercial Lines | Commercial Property Segment
Segment Reporting Information [Line Items]
Revenues	161	138	460	403
Commercial Lines | Commercial Auto Segment
Segment Reporting Information [Line Items]
Revenues	124	108	355	315
Commercial Lines | Workers' Compensation
Segment Reporting Information [Line Items]
Revenues	95	89	270	255
Commercial Lines | Specialty Packages
Segment Reporting Information [Line Items]
Revenues	38	37	114	112
Commercial Lines | Surety and Executive Risk
Segment Reporting Information [Line Items]
Revenues	30	28	89	82
Commercial Lines | Machinery and Equipment
Segment Reporting Information [Line Items]
Revenues	11	10	32	29
Commercial Lines | Commercial Lines Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	680	607	1,956	1,765
Commercial Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	1	0	2	2
Personal Lines
Segment Reporting Information [Line Items]
Revenues	244	220	713	644
Income (loss) before income taxes	14	(12)	33	(89)
Personal Lines | Personal Auto
Segment Reporting Information [Line Items]
Revenues	112	101	328	299
Personal Lines | Home Owner Segment
Segment Reporting Information [Line Items]
Revenues	103	90	298	261
Personal Lines | Other Personal Lines
Segment Reporting Information [Line Items]
Revenues	29	28	86	82
Personal Lines | Commercial Lines Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	244	219	712	642
Personal Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	0	1	1	2
Excess And Surplus Lines Insurance
Segment Reporting Information [Line Items]
Revenues	30	25	85	68
Income (loss) before income taxes	4	(3)	5	(8)
Life Insurance Segment
Segment Reporting Information [Line Items]
Revenues	39	39	127	131
Income (loss) before income taxes	0	0	10	(2)
Identifiable assets	1,263		1,263		1,201
Life Insurance Segment | Life Insurance Excluding Account Investment Fees
Segment Reporting Information [Line Items]
Revenues	38	38	124	130
Life Insurance Segment | Seperate Account Investment Management Fees
Segment Reporting Information [Line Items]
Revenues	1	1	3	1
Investments Segment
Segment Reporting Information [Line Items]
Revenues	155	142	469	424
Income (loss) before income taxes	134	121	409	362
Identifiable assets	13,137		13,137		12,599
Investments Segment | Investment Income, Net
Segment Reporting Information [Line Items]
Net Investment Income	133	132	392	395
Investments Segment | Realized Investment Gains, Net
Segment Reporting Information [Line Items]
Realized gains on investments	22	10	77	29
Other
Segment Reporting Information [Line Items]
Revenues	3	2	7	7
Income (loss) before income taxes	(13)	(14)	(45)	(44)
Identifiable assets	$ 460		$ 460		$ 353