Segment Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenues	$ 1,189	$ 1,103
Net Investment Income	135	128
Realized gains on investments	22	41
Income (loss) before income taxes	119	217
Identifiable assets	17,942		17,662
Property Casualty Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	2,493		2,455
Commercial Lines
Segment Reporting Information [Line Items]
Revenues	693	631
Income (loss) before income taxes	2	58
Commercial Lines | Commercial Casualty Segment
Segment Reporting Information [Line Items]
Revenues	224	204
Commercial Lines | Commercial Property Segment
Segment Reporting Information [Line Items]
Revenues	171	147
Commercial Lines | Commercial Auto Segment
Segment Reporting Information [Line Items]
Revenues	126	114
Commercial Lines | Workers' Compensation
Segment Reporting Information [Line Items]
Revenues	92	88
Commercial Lines | Specialty Packages
Segment Reporting Information [Line Items]
Revenues	36	39
Commercial Lines | Management liability and surety
Segment Reporting Information [Line Items]
Revenues	31	29
Commercial Lines | Machinery and Equipment
Segment Reporting Information [Line Items]
Revenues	12	10
Commercial Lines | Commercial Lines Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	692	631
Commercial Lines | Other Fee Revenue
Segment Reporting Information [Line Items]
Revenues	1	0
Personal Lines
Segment Reporting Information [Line Items]
Income (loss) before income taxes	(7)	20
Personal Lines | Personal Auto
Segment Reporting Information [Line Items]
Revenues	116	107
Personal Lines | Home Owner Segment
Segment Reporting Information [Line Items]
Revenues	109	96
Personal Lines | Other Personal Lines
Segment Reporting Information [Line Items]
Revenues	29	28
Personal Lines | Commercial Lines Insurance Premiums
Segment Reporting Information [Line Items]
Revenues	254	231
Excess And Surplus Lines Insurance
Segment Reporting Information [Line Items]
Revenues	33	27
Income (loss) before income taxes	4	0
Life Insurance Segment
Segment Reporting Information [Line Items]
Revenues	50	43
Income (loss) before income taxes	0	7
Identifiable assets	1,302		1,225
Life Insurance Segment | Life Insurance Excluding Account Investment Fees
Segment Reporting Information [Line Items]
Revenues	48	42
Life Insurance Segment | Seperate Account Investment Management Fees
Segment Reporting Information [Line Items]
Revenues	2	1
Investments Segment
Segment Reporting Information [Line Items]
Revenues	157	169
Income (loss) before income taxes	136	148
Identifiable assets	13,730		13,618
Investments Segment | Investment Income, Net
Segment Reporting Information [Line Items]
Net Investment Income	135	128
Investments Segment | Realized Investment Gains, Net
Segment Reporting Information [Line Items]
Realized gains on investments	22	41
Other
Segment Reporting Information [Line Items]
Revenues	2	2
Income (loss) before income taxes	(16)	(16)
Identifiable assets	$ 417		$ 364