| **Investments (Narrative) (Details) (USD $)In Millions, unless otherwise specified**                              | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 |
|-------------------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|
| **Investments [Abstract]**                                                                                        |          |          |          |
| Number of non-income producing fixed maturity securities                                                          |     | 9             |          |
| Fair value of non income fixed maturity security                                                                  |          | $ 1           |          |
| Number of investments that exceed ten percent of stockholders' equity                                             |     |     |          |
| Limited partnership investments                                                                                   | 2,720         | 2,462         |          |
| Undistributed earnings of limited partnership investments                                                         | 969           | 768           |          |
| Percentage of limited partnerships reported on a current basis                                                    | 67.00%        |          |          |
| Percentage of limited partnerships reported on a one month lag                                                    | 16.00%        |          |          |
| Number of active limited partnership investments held                                                             | 87            | 79            |          |
| Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments | 74.00%        | 80.00%        |          |
| Percentage of equity related limited partnership hedge fund strategies                                            | 51.00%        |          |          |
| Percentage of multistrategy approach limited partnership hedge fund strategies                                    | 28.00%        |          |          |
| Percentage of distressed investments limited partnership hedge fund strategies                                    | 17.00%        |          |          |
| Percentage of fixed income limited partnership hedge fund strategies                                              | 4.00%         |          |          |
| Limited partnerships invested in private debt and equity                                                          | 22.00%        | 16.00%        |          |
| Carrying value of ten largest limited partnerships                                                                | 1,471         | 1,309         |          |
| 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%         | 3.00%         | 4.00%         |
| Commercial mortgage loans past due or in non-accrual status                                                       |     |     |          |
| Valuation allowances recorded on commercial mortgage loans                                                        |     |     |          |
| Future capital call commitments for limited partnership investments                                               | 381           |          |          |
| Commitments to purchase or fund privately placed debt securities                                                  | 151           |          |          |
| Commitments to sell various privately placed debt securities                                                      | 145           |          |          |
| Carrying value of securities deposited under requirements of regulatory authorities                               | 3,300         | 3,600         |          |
| Cash and securities deposited as collateral for letters of credit                                                 | 353           | 4             |          |
| Carrying value of assets deposited to secure reinsurance and third party obligations                              | $ 294         | $ 277         |          |

| X                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying Value of Assets Deposited to Secure Reinsurance and Third Party Obligations
+ References
No definition available.
+ Details**Name:**cna_CarryingValueOfAssetsDepositedToSecureReinsuranceAndThirdPartyObligations**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying value Of Limited Partnerships As Percentage of Aggregate Partnership Equity
+ References
No definition available.
+ Details**Name:**cna_CarryingValueOfLimitedPartnershipsAsPercentageOfAggregatePartnershipEquity**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying Value Of Securities Deposited Under Requirements Of Regulatory Authorities
+ References
No definition available.
+ Details**Name:**cna_CarryingValueOfSecuritiesDepositedUnderRequirementsOfRegulatoryAuthorities**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying Value Of Ten Largest Limited Partnerships
+ References
No definition available.
+ Details**Name:**cna_CarryingValueOfTenLargestLimitedPartnerships**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cash And Securities Deposited As Collateral For Letters Of Credit
+ References
No definition available.
+ Details**Name:**cna_CashAndSecuritiesDepositedAsCollateralForLettersOfCredit**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Commitments To Purchase Or Fund Additional Amounts
+ References
No definition available.
+ Details**Name:**cna_CommitmentsToPurchaseOrFundAdditionalAmounts**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Commitments To Sell Various Privately Placed Debt Securities
+ References
No definition available.
+ Details**Name:**cna_CommitmentsToSellVariousPrivatelyPlacedDebtSecurities**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Of Non Income Fixed Maturity Security
+ References
No definition available.
+ Details**Name:**cna_FairValueOfNonIncomeFixedMaturitySecurity**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Future capital call commitments to limited partnerships for which we would receive an ownership interest.
+ References
No definition available.
+ Details**Name:**cna_FutureCapitalCallCommitments**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Income As Percentage Of Change In Partnership Equity For All Limited Partnerships
+ References
No definition available.
+ Details**Name:**cna_IncomeAsPercentageOfChangeInPartnershipEquityForAllLimitedPartnerships**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the carrying amount on the entity's balance sheet of its investment in limited partnerships.
+ References
No definition available.
+ Details**Name:**cna_LimitedPartnershipInvestments**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Limited Partnerships Invested In Private Debt and Equity
+ References
No definition available.
+ Details**Name:**cna_LimitedPartnershipsInvestedInPrivateDebtAndEquity**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of active limited partnership investments held
+ References
No definition available.
+ Details**Name:**cna_NumberOfActiveLimitedPartnershipInvestmentsHeld**Namespace Prefix:**cna_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Investments that Exceed Ten Percent of Stockholders' Equity, other than investments in U.S. Treasury and U.S. Government agency securities.
+ References
No definition available.
+ Details**Name:**cna_NumberOfInvestmentsThatExceedTenPercentOfStockholdersEquity**Namespace Prefix:**cna_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Non-Income Producing Fixed Maturity Securities
+ References
No definition available.
+ Details**Name:**cna_NumberOfNonIncomeProducingFixedMaturitySecurities**Namespace Prefix:**cna_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Distressed Investments Limited Partnership Hedge Fund Strategies
+ References
No definition available.
+ Details**Name:**cna_PercentageOfDistressedInvestmentsLimitedPartnershipHedgeFundStrategies**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Equity Related Limited Partnership Hedge Fund Strategies
+ References
No definition available.
+ Details**Name:**cna_PercentageOfEquityRelatedLimitedPartnershipHedgeFundStrategies**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Fixed Income Limited Partnership Hedge Fund Strategies
+ References
No definition available.
+ Details**Name:**cna_PercentageOfFixedIncomeLimitedPartnershipHedgeFundStrategies**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments
+ References
No definition available.
+ Details**Name:**cna_PercentageOfLimitedPartnershipsEmployingHedgeFundStrategiesFocusedOnFixedIncomeAndEquityInvestments**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of total carrying value of limited partnerships reported on a current basis
+ References
No definition available.
+ Details**Name:**cna_PercentageOfLimitedPartnershipsReportedOnCurrentBasis**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of total carrying value of limited partnerships reported on a one month lag.
+ References
No definition available.
+ Details**Name:**cna_PercentageOfLimitedPartnershipsReportedOnOneMonthLag**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Multistrategy Approach Limited Partnership Hedge Fund Strategies
+ References
No definition available.
+ Details**Name:**cna_PercentageOfMultistrategyApproachLimitedPartnershipHedgeFundStrategies**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A valuation allowance for financing receivables that are expected to be uncollectible.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=28368275&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLosses**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded Investment in financing receivables that are past due at the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=28368275&loc=SL6953401-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentPastDue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 852 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6462270&loc=d3e57205-112772
+ Details**Name:**us-gaap_RetainedEarningsUndistributedEarningsFromEquityMethodInvestees**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |