Segment Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenues	$ 1,214	$ 1,104	$ 2,403	$ 2,207
Net Investment Income	136	131	271	259
Realized gains on investments	14	14	36	55
Income (loss) before income taxes	107	148	226	365
Identifiable assets	18,335		18,335		17,662
Property Casualty Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	2,636		2,636		2,455
Commercial Lines
Segment Reporting Information [Line Items]
Revenues	715	646	1,408	1,277
Income (loss) before income taxes	28	34	30	92
Commercial Lines | Commercial Casualty Segment
Segment Reporting Information [Line Items]
Revenues	234	211	458	415
Commercial Lines | Commercial Property Segment
Segment Reporting Information [Line Items]
Revenues	180	152	351	299
Commercial Lines | Commercial Auto Segment
Segment Reporting Information [Line Items]
Revenues	132	117	258	231
Commercial Lines | Workers' Compensation
Segment Reporting Information [Line Items]
Revenues	95	87	187	175
Commercial Lines | Specialty Packages
Segment Reporting Information [Line Items]
Revenues	30	37	66	76
Commercial Lines | Management liability and surety
Segment Reporting Information [Line Items]
Revenues	31	30	62	59
Commercial Lines | Machinery and Equipment
Segment Reporting Information [Line Items]
Revenues	12	11	24	21
Commercial Lines | Commercial Lines Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	714	645	1,406	1,276
Commercial Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	1	1	2	1
Personal Lines
Segment Reporting Information [Line Items]
Revenues	259	238	513	469
Income (loss) before income taxes	(40)	(1)	(47)	19
Personal Lines | Personal Auto
Segment Reporting Information [Line Items]
Revenues	117	109	233	216
Personal Lines | Home Owner Segment
Segment Reporting Information [Line Items]
Revenues	111	99	220	195
Personal Lines | Other Personal Lines
Segment Reporting Information [Line Items]
Revenues	30	29	59	57
Personal Lines | Commercial Lines Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	258	237	512	468
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	34	28	67	55
Income (loss) before income taxes	5	1	9	1
Life Insurance Segment
Segment Reporting Information [Line Items]
Revenues	54	45	104	88
Income (loss) before income taxes	(1)	3	(1)	10
Identifiable assets	1,263		1,263		1,225
Life Insurance Segment | Life Insurance Excluding Account Investment Fees
Segment Reporting Information [Line Items]
Revenues	53	44	101	86
Life Insurance Segment | Seperate Account Investment Management Fees
Segment Reporting Information [Line Items]
Revenues	1	1	3	2
Investments Segment
Segment Reporting Information [Line Items]
Revenues	150	145	307	314
Income (loss) before income taxes	130	127	266	275
Identifiable assets	14,104		14,104		13,618
Investments Segment | Investment Income, Net
Segment Reporting Information [Line Items]
Net Investment Income	136	131	271	259
Investments Segment | Realized Investment Gains, Net
Segment Reporting Information [Line Items]
Realized gains on investments	14	14	36	55
Other
Segment Reporting Information [Line Items]
Revenues	2	2	4	4
Income (loss) before income taxes	(15)	(16)	(31)	(32)
Identifiable assets	$ 332		$ 332		$ 364