| **Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions**          | 3 Months Ended | 9 Months Ended | 48 Months Ended |         |        |
|--------------------------------------------------------------------------------------------------------|----------------|----------------|-----------------|---------|--------|
||
| Catastrophe losses net of reinsurance                                                                  | $ 16           |           | $ 14            | $ 137   | $ 103  |   |   |   |
| Loss Portfolio Transfer [Member]                                                                       |           |           |            |    |   |   |   |   |
| Liability for Asbestos and Environmental Claims, Net                                                   |           |           |            |    |   |   |   | $ 1,600 |
| Aggregate limit under AEP Loss Portfolio Transfer                                                      |           |           |            |    |   |   |   | 4,000 |
| Claim And Allocated Claim Adjustment Expense Reserves Under Existing Third Party Reinsurance Contracts |           |           |            |    |   |   |   | 1,200 |
| Reinsurance premium paid to NICO under AEP Loss Portfolio Transfer                                     |           |           |            |    |   |   |   | 2,000 |
| Net reinsurance receivables transferrred to NICO under AEP Loss Portfolio Transfer                     |           |           |            |    |   |   |   | 215 |
| Total consideration paid to NICO under AEP Loss Portfolio Transfer                                     |           |           |            |    |   |   |   | $ 2,200 |
| Net AEP adverse development before consideration of LPT                                                | 0              | $ 200          | $ 0             | 200     | $ 150  | $ 900 |   |   |
| Cumulative amounts ceded under AEP Loss Portfolio Transfer                                             | 2,800          |           |            | 2,800   |   |   | $ 2,600 |   |
| Deferred reinsurance benefit yet to be recognized                                                      | 347            |           |            | 347     |   |   | 241 |   |
| Fair value of collateral trust account established by NICO under AEP Loss Portfolio Transfer           | $ 2,500        |           |            | $ 2,500 |   |   | $ 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 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
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
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=d3e14764-158437
+ 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:** |