| **Claim and Claim Adjustment Expense Reserves (Narrative) (Details) (USD $)In Millions, unless otherwise specified**                                                             | 12 Months Ended | 36 Months Ended |        |        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|--------|--------|
||
| Catastrophe losses net of reinsurance                                                                                                                                            | $ 391           | $ 222           | $ 121  |   |   |   |   |
| Net A&EP claim and allocated claim adjustment expense reserves ceded to NICO                                                                                                 |            |            |   |   |   |   | 1,600 |
| Aggregate limit under A&EP Loss Portfolio Transfer                                                                                                                           |            |            |   |   |   |   | 4,000 |
| Net claim and allocated claim adjustment expense reserves in discontinued operations ceded to NICO under A&EP Loss Portfolio Transfer                                        |            |            |   |   |   |   | 90 |
| A&EP claim and allocated claim adjustment expense reserves ceded under existing third party reinsurance contracts transferred to NICO under A&EP Loss Portfolio Transfer |            |            |   |   |   |   | 1,200 |
| Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer                                                                                                          |            |            |   |   |   | 2,000 |   |
| Net reinsurance receivables transferrred to NICO under A&EP Loss Portfolio Transfer                                                                                          |            |            |   |   |   |   | 215 |
| Allowance for doubtful accounts related to reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer                                                    |            |            |   |   |   |   | 100 |
| Collateral trust established by NICO under A&EP Loss Portfolio Transfer                                                                                                      |            |            |   |   |   | 2,200 |   |
| Reduction of allowance for uncollectible reinsurance receivables                                                                                                                 |            |            |   | 200    |   |   |   |
| Remaining amount available under aggregate limit of the Loss Portfolio Transfer on an incurred basis                                                                             |            |            |   |   | 2,000 |   |   |
| (Unfavorable) favorable net prior year development                                                                                                                               | 205             | 423             | 637    |   | (399) |   |   |
| Future adverse prior year development to result in retroactive reinsurance gain                                                                                                  |            |            |   |   | 230 |   |   |
| Net ultimate paid losses ceded under the Loss Portfolio Transfer                                                                                                                 |            |            |   |   | 661 |   |   |
| Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer                                                                                |            |            |   |   | 2,500 |   |   |
| (Unfavorable) favorable net prior year development, Life & Group                                                                                                             | $ 11            | $ 29            | $ 2    |   |   |   |   |

| X                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate limit under A&EP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_AggregateLimitUnderAepLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Allowance for doubtful accounts related to reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_AllowanceForDoubtfulAccountsRelatedToReinsuranceReceivablesTransferredToNicoUnderAepLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Catastrophe losses net of reinsurance
+ References
No definition available.
+ Details**Name:**cna_CatastropheLossesNetOfReinsurance**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Claim and allocated claim adjustment expense reserves under existing third party reinsurance contracts.
+ References
No definition available.
+ Details**Name:**cna_ClaimAndAllocatedClaimAdjustmentExpenseReservesUnderExistingThirdPartyReinsuranceContracts**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Collateral trust established by NICO under A&EP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_CollateralTrustEstablishedByNicoUnderAepLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_FairValueOfCollateralTrustAccountEstablishedByNicoUnderAepLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Future adverse prior year development to result in retroactive reinsurance gain
+ References
No definition available.
+ Details**Name:**cna_FutureAdversePriorYearDevelopmentToResultInRetroactiveReinsuranceGain**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net claim and allocated claim adjustment expense reserves in discontinued operations ceded to NICO
+ References
No definition available.
+ Details**Name:**cna_NetClaimAndAllocatedClaimAdjustmentExpenseReservesInDiscontinuedOperationsCededToNico**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net reinsurance receivables transferrred to NICO under A&EP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_NetReinsuranceReceivablesTransferrredToNicoUnderAepLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net ultimate paid losses ceded under the Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_NetUltimatePaidLossesCededUnderLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group
+ References
No definition available.
+ Details**Name:**cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development Life & Group
+ References
No definition available.
+ Details**Name:**cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentLifeGroup**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reduction of allowance for uncollectible reinsurance receivables
+ References
No definition available.
+ Details**Name:**cna_ReductionOfAllowanceForUncollectibleReinsuranceReceivables**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_ReinsurancePremiumPaidToNicoUnderAepLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Remaining amount available under aggregate limit of the Loss Portfolio Transfer on an incurred basis
+ References
No definition available.
+ Details**Name:**cna_RemainingAmountAvailableUnderAggregateLimitOfLossPortfolioTransferOnIncurredBasis**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, net of estimated reinsurance recoveries.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-PLI -Paragraph 132, 133, 135, 136, 137 -IssueDate 2006-09-01 -Chapter 4 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11, 12 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
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