Investments (Narrative) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investments [Abstract]
Number of non-income producing fixed maturity securities	0	0
Number of investments that exceed ten percent of stockholders' equity	0	0
Limited partnership investments	$ 2,937,000,000cna_LimitedPartnershipInvestments	$ 2,720,000,000cna_LimitedPartnershipInvestments
Undistributed earnings of limited partnership investments	1,040,000,000	969,000,000
Percentage of limited partnerships reported on a current basis	65.00%
Percentage of limited partnerships reported on a one month lag	16.00%
Number of active limited partnership investments held	86	87
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments	73.00%	74.00%
Limited partnerships invested in private debt and equity	23.00%	22.00%
Percentage of equity related limited partnership hedge fund strategies	56.00%
Percentage of multistrategy approach limited partnership hedge fund strategies	27.00%
Percentage of distressed investments limited partnership hedge fund strategies	14.00%
Percentage of fixed income limited partnership hedge fund strategies	3.00%
Carrying value of ten largest limited partnerships	1,492,000,000	1,471,000,000
Carrying value of limited partnerships as percentage of aggregate partnership equity	4.00%	4.00%
Income As Percentage Of Change In Partnership Equity For All Limited Partnerships	4.00%	4.00%	3.00%
Commercial mortgage loans past due or in non-accrual status	0	0
Valuation allowances recorded on commercial mortgage loans	0	0
Future capital call commitments for limited partnership investments	327,000,000cna_FutureCapitalCallCommitments
Mortgage loan commitments	8,000,000cna_MortgageLoanCommitments
Commitments to purchase or fund privately placed debt securities	75,000,000
Commitments to sell various privately placed debt securities	97,000,000
Carrying value of securities deposited under requirements of regulatory authorities	3,000,000,000	3,300,000,000
Cash and securities deposited as collateral for letters of credit	361,000,000	353,000,000
Carrying value of assets deposited to secure reinsurance and third party obligations	$ 302,000,000	$ 294,000,000

Carrying Value of Assets Deposited to Secure Reinsurance and Third Party Obligations

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Carrying value Of Limited Partnerships As Percentage of Aggregate Partnership Equity

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Carrying Value Of Securities Deposited Under Requirements Of Regulatory Authorities

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Carrying Value Of Ten Largest Limited Partnerships

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Cash And Securities Deposited As Collateral For Letters Of Credit

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Commitments To Purchase Or Fund Additional Amounts

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Commitments To Sell Various Privately Placed Debt Securities

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Future capital call commitments to limited partnerships for which we would receive an ownership interest.

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Income As Percentage Of Change In Partnership Equity For All Limited Partnerships

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This item represents the carrying amount on the entity's balance sheet of its investment in limited partnerships.

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Limited Partnerships Invested In Private Debt and Equity

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Mortgage loan commitments, representing signed loan applications received and accepted.

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Number of active limited partnership investments held

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Number of Investments that Exceed Ten Percent of Stockholders' Equity, other than investments in U.S. Treasury and U.S. Government agency securities.

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Number of Non-Income Producing Fixed Maturity Securities

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Percentage of Distressed Investments Limited Partnership Hedge Fund Strategies

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Percentage of Equity Related Limited Partnership Hedge Fund Strategies

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Percentage of Fixed Income Limited Partnership Hedge Fund Strategies

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Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments

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Percentage of total carrying value of limited partnerships reported on a current basis

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Percentage of total carrying value of limited partnerships reported on a one month lag.

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Percentage of Multistrategy Approach Limited Partnership Hedge Fund Strategies

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A valuation allowance for financing receivables that are expected to be uncollectible.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

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Recorded Investment in financing receivables that are past due at the balance sheet date.

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Amount of consolidated retained earnings that represent undistributed (not yet received) earnings from 50% or less owned persons accounted for by the equity method (equity method investees).

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

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