Segments Information - Revenues and Income Before Income Tax of Each Operating Segment (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
Revenues
Premiums earned related to insurance business	$ 2,994	$ 2,983	$ 5,999	$ 5,931
Investment Income	368	390	740	791
Premiums earned related to reinsurance assumed	1			3
Net Premiums earned	2,995	2,983	5,999	5,934
Corporate and other	16	12	28	24
Realized investment gains, net	179	47	317	103
Total revenues	3,551	3,423	7,068	6,833
Insurance underwriting income (loss)	295	141	735	419
Reinsurance assumed underwriting income	6	8	10	19
Underwriting income before deferred policy acquisition costs	301	149	745	438
Increase in deferred policy acquisition costs	16	10	57	24
Underwriting income	317	159	802	462
Total property and casualty insurance	675	538	1,516	1,220
Corporate and other	(58)	(58)	(121)	(117)
Realized investment gains, net	179	47	317	103
Income Before Federal and Foreign Income Tax	796	527	1,712	1,206
Personal insurance [Member]
Revenues
Premiums earned related to insurance business	1,050	1,014	2,081	2,004
Insurance underwriting income (loss)	75	58	225	219
Commercial insurance [Member]
Revenues
Premiums earned related to insurance business	1,299	1,305	2,607	2,591
Insurance underwriting income (loss)	124	18	319	69
Specialty insurance [Member]
Revenues
Premiums earned related to insurance business	645	664	1,311	1,336
Insurance underwriting income (loss)	96	65	191	131
Property and casualty insurance [Member]
Revenues
Investment Income	361	381	724	772
Total revenues	3,356	3,364	6,723	6,706
Investment income	349	373	700	753
Other income	$ 9	$ 6	$ 14	$ 5

Represents income (loss) before income tax related to non-insurance companies before realized investment gain (loss).

No definition available.

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Represents revenues related to non-insurance companies, before realized investment gains or losses.

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No definition available.

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Reflects the sum of all other income and charges recognized by the entity in the period not otherwise specified separately.

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No definition available.

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- Definition

Premiums recognized as revenue in the period earned on all property and casualty insurance and reinsurance contracts after subtracting any amounts ceded to another insurer and before reinsurance assumed business which is in run-off.

+ References

No definition available.

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Premiums recognized as revenue in the period earned on property and casualty reinsurance assumed business which is in run-off, after subtracting any amounts ceded to another insurer.

+ References

No definition available.

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- Definition

Amount of income (loss) before income tax for property and casualty insurance including underwriting income (loss), investment income (loss) and other income (charges), before realized investment gain (loss).

+ References

No definition available.

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- Definition

Amount of underwriting income (loss) for the reportable property and casualty insurance segments which is arrived at by reducing premiums earned by losses and loss expenses incurred and statutory underwriting expenses incurred, before reinsurance assumed business which is in run-off. Under statutory accounting principles, policy acquisition and other underwriting expenses are recognized immediately, not at the time premiums are earned.

+ References

No definition available.

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Amount of underwriting income (loss) for the property and casualty insurance reinsurance assumed segment, which is in run-off, which is arrived at by reducing premiums earned by losses and loss expenses incurred and statutory underwriting expenses incurred. Under statutory accounting principles, policy acquisition and other underwriting expenses are recognized immediately, not at the time premiums are earned.

+ References

No definition available.

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Property And Casualty Underwriting Income Loss

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Property And Casualty Underwriting Income Loss Before Deferred Policy Acquisition Costs

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This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.7(c),9(a))

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-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (c)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Article 5

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-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (c)-(e)

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(h)(1)(i))

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- Definition

The increase (decrease) during the reporting period in the balance sheet value of capitalized sales costs that are associated with acquiring a new insurance customers.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

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Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.1-5)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1, 2 , 3, 4, 5

-Article 9

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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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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 16

-Article 12

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-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.2)

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-Number 210

-Section 04

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-Article 7

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

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.1)

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-Number 210

-Section 04

-Paragraph 1

-Article 7

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-Topic 944

-SubTopic 605

-Section 45

-Paragraph 1

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-Name Accounting Standards Codification

-Topic 944

-SubTopic 605

-Section 50

-Paragraph 1

-Subparagraph (b),(c)

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

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-Name Regulation S-X (SX)

-Number 210

-Section 03

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-Article 5

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