Segment Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Earned premiums	$ 1,094us-gaap_PremiumsEarnedNet	$ 1,027us-gaap_PremiumsEarnedNet
Fee revenues	3us-gaap_FeesAndCommissions	3us-gaap_FeesAndCommissions
Investment income, net of expenses	139us-gaap_NetInvestmentIncome	135us-gaap_NetInvestmentIncome
Realized investment gains, net	47us-gaap_RealizedInvestmentGainsLosses	22us-gaap_RealizedInvestmentGainsLosses
Other	2us-gaap_OtherIncome	2us-gaap_OtherIncome
Revenues	1,285us-gaap_Revenues	1,189us-gaap_Revenues
Income (loss) before income taxes	174	119
Identifiable assets	18,897us-gaap_Assets		18,753us-gaap_Assets
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information [Line Items]
Identifiable assets	2,711		2,656
Life Insurance Segment
Segment Reporting Information [Line Items]
Identifiable assets	1,306		1,316
Investments Segment
Segment Reporting Information [Line Items]
Identifiable assets	14,544		14,441
Operating Segments \| Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	733	692
Fee revenues	1	1
Revenues	734	693
Income (loss) before income taxes	26	2
Operating Segments \| Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	268	254
Fee revenues	1	0
Revenues	269	254
Income (loss) before income taxes	(3)	(7)
Operating Segments \| Excess and Surplus Lines Insurance
Segment Reporting Information [Line Items]
Earned premiums	40	33
Income (loss) before income taxes	4	4
Operating Segments \| Life Insurance Segment
Segment Reporting Information [Line Items]
Earned premiums	53	48
Fee revenues	1	2
Revenues	54	50
Income (loss) before income taxes	(3)	0
Operating Segments \| Investments Segment
Segment Reporting Information [Line Items]
Investment income, net of expenses	139	135
Realized investment gains, net	47	22
Revenues	186	157
Income (loss) before income taxes	165	136
Corporate and Other Non-Segment
Segment Reporting Information [Line Items]
Other	2	2
Income (loss) before income taxes	(15)	(16)
Identifiable assets	336		340
Commercial Casualty \| Operating Segments \| Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	244	224
Commercial Property \| Operating Segments \| Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	196	171
Commercial Auto \| Operating Segments \| Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	136	126
Workers' Compensation \| Operating Segments \| Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	93	92
Other Commercial Lines \| Operating Segments \| Commercial Lines
Segment Reporting Information [Line Items]
Earned premiums	64	79
Personal Auto \| Operating Segments \| Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	123	116
Home Owner Segment \| Operating Segments \| Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	114	109
Other Personal Lines \| Operating Segments \| Personal Lines
Segment Reporting Information [Line Items]
Earned premiums	$ 31	$ 29

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of fees and commissions from financial services and banking activities and correspondent clearing. Includes fees from depositor accounts, credit cards, merchant discounts, and fiduciary and trust activities, guarantee fees, investment advisory, management, and administrative fees, transfer agent fees, and insurance commissions.

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Sum of operating profit and nonoperating income or expense before Income or Loss from equity method investments, income taxes, extraordinary items, and noncontrolling interest.

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

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