| **Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions**                                                                                    | 3 Months Ended | 12 Months Ended |        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|-----------------|--------|
||
| Catastrophe losses net of reinsurance                                                                                                                                            |           | $ 165           | $ 141  | $ 156 |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development Life & Group                                                  |           | (45)            | (50)   | (14) |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Workers' Compensation Reinsurance Pool                                                          |           |            |   | 26 |   |
| Loss Portfolio Transfer [Member]                                                                                                                                                 |           |            |   |   |   |
| 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 |
| 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 |
| Total consideration paid to NICO under AEP Loss Portfolio Transfer                                                                                                               |           |            |   |   | $ 2,200 |
| Net AEP adverse development before consideration of LPT                                                                                                                          | $ 200          | 200             | 150    | $ 0 |   |
| Unfavorable Development related to Asbestos Claims and Higher than Expected Severity in Pollution Claims                                                                         |           |            | 150    |   |   |
| Cumulative amounts ceded under AEP Loss Portfolio Transfer                                                                                                                       |           | 2,800           | 2,600  |   |   |
| Deferred reinsurance benefit yet to be recognized                                                                                                                                |           | 334             | $ 241  |   |   |
| Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer                                                                                |           | $ 2,800         |   |   |   |

| 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
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
Cumulative amounts ceded under AEP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_CumulativeamountscededunderAEPLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Deferred reinsurance benefit yet to be recognized
+ References
No definition available.
+ Details**Name:**cna_Deferredreinsurancebenefityettoberecognized**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**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
Net AEP adverse development before consideration of LPT
+ References
No definition available.
+ Details**Name:**cna_NetAEPadversedevelopmentbeforeconsiderationofLPT**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Workers' Compensation Reinsurance Pool
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentWorkersCompensationReinsurancePool**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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
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:**debit**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
Total consideration paid to NICO under AEP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_TotalconsiderationpaidtoNICOunderAEPLossPortfolioTransfer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unfavorable Development related to Asbestos Claims and Higher than Expected Severity in Pollution Claims
+ References
No definition available.
+ Details**Name:**cna_UnfavorableDevelopmentrelatedtoAsbestosClaimsandHigherthanExpectedSeverityinPollutionClaims**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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 (cc) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437Reference 2: 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=65670824&loc=d3e14764-158437
+ Details**Name:**us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementScenarioAxis=cna_LossPortfolioTransferMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |