| **Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions**          | 3 Months Ended | 6 Months Ended |        |        |
|--------------------------------------------------------------------------------------------------------|----------------|----------------|--------|--------|
||
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                    |           |           |   |   |   |   |
| Unusual or infrequent item, or both, net of insurance proceeds                                         | $ 301          | $ 38           | $ 376  | $ 96   |   |   |
| Retroactive reinsurance benefit recognized                                                             | 20             | $ 14           | 34     | $ (36) |   |   |
| Asbestos and Environmental Reserves                                                                    |           |           |   |   |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                    |           |           |   |   |   |   |
| 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 transferred to NICO under AEP Loss Portfolio Transfer                      |           |           |   |   |   | 215 |
| Total consideration paid to NICO under AEP Loss Portfolio Transfer                                     |           |           |   |   |   | $ 2,200 |
| Cumulative amounts ceded under AEP Loss Portfolio Transfer                                             | 3,200          |           | 3,200  |   | $ 3,200 |   |
| Deferred reinsurance benefit yet to be recognized                                                      | 358            |           | 358    |   | 392 |   |
| Fair value of collateral trust account established by NICO under AEP Loss Portfolio Transfer           | 3,300          |           | 3,300  |   | $ 3,700 |   |
| COVID-19                                                                                               |           |           |   |   |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                    |           |           |   |   |   |   |
| Unusual or infrequent item, or both, net of insurance proceeds                                         | 182            |           | 195    |   |   |   |
| Civil Unrest [Member]                                                                                  |           |           |   |   |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                    |           |           |   |   |   |   |
| Unusual or infrequent item, or both, net of insurance proceeds                                         | 61             |           | 61     |   |   |   |
| Weather Related Events                                                                                 |           |           |   |   |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                    |           |           |   |   |   |   |
| Unusual or infrequent item, or both, net of insurance proceeds                                         | $ 58           |           | $ 120  |   |   |   |

| 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
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 reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer
+ References
No definition available.
+ Details**Name:**cna_NetreinsurancereceivablestransferredtoNICOunderAEPLossPortfolioTransfer**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
Retroactive reinsurance benefit recognized
+ References
No definition available.
+ Details**Name:**cna_Retroactivereinsurancebenefitrecognized**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437
+ Details**Name:**us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after deduction of insurance proceeds, of loss recognized in the income statement for an event or transaction that is unusual in nature or infrequent in occurrence, or both.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 930 -SubTopic 715 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109241964&loc=d3e23713-110274Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=114868883&loc=SL114871943-224233
+ Details**Name:**us-gaap_UnusualOrInfrequentItemNetOfInsuranceProceeds**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cna_AsbestosandEnvironmentalReservesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=cna_COVID19Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=cna_CivilUnrestMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=cna_WeatherRelatedEventsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |