Segments Information - Revenues and Income Before Income Tax of Each Operating Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Revenues
Net Premiums earned	$ 3,105	$ 3,029
Investment Income	339	358
Corporate and other	9cb_CorporateAndOtherRevenues	10cb_CorporateAndOtherRevenues
Realized investment gains, net	11us-gaap_GainLossOnInvestments	116us-gaap_GainLossOnInvestments
Total Revenues	3,457us-gaap_Revenues	3,506us-gaap_Revenues
Insurance underwriting income (loss)	188	195
Increase in deferred policy acquisition costs	17	13
Underwriting income	205	208
Total property and casualty insurance	533	547
Corporate and other	(62)cb_CorporateAndOtherIncomeLoss	(60)cb_CorporateAndOtherIncomeLoss
Realized investment gains, net	11us-gaap_GainLossOnInvestments	116us-gaap_GainLossOnInvestments
Income Before Federal and Foreign Income Tax	482	603
Property and Casualty Insurance [Member]
Revenues
Investment Income	332	351
Total Revenues	3,437	3,380
Investment income	321	341
Other income (charges)	7	(2)
Operating Segments [Member] \| Personal Insurance [Member]
Revenues
Premiums earned related to insurance business	1,113	1,076
Insurance underwriting income (loss)	(30)	2
Operating Segments [Member] \| Commercial Insurance [Member]
Revenues
Premiums earned related to insurance business	1,342	1,300
Insurance underwriting income (loss)	91	111
Operating Segments [Member] \| Specialty Insurance [Member]
Revenues
Premiums earned related to insurance business	650	653
Insurance underwriting income (loss)	$ 127	$ 82

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

No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

Other income loss

+ References

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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- 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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- Definition

The amount of property and casualty underwriting income (loss).

+ References

No definition available.

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- Definition

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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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (c)

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (c)-(e)

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Article 5

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- Definition

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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

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- Definition

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.

+ References

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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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- Definition

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.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 16

-Article 12

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.2)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 2

-Article 7

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- Definition

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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.1)

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-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1

-Article 7

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-Name Accounting Standards Codification

-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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- Definition

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

-Paragraph 1

-Article 5

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