| **Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions**                                                                                    | 3 Months Ended | 12 Months Ended |         |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|-----------------|---------|
||
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                              |           |            |    |   |   |
| Catastrophe losses net of reinsurance                                                                                                                                            |           | $ 380           | $ 165   | $ 141 |   |
| Payments for Reinsurance                                                                                                                                                         |           | 4               |    |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development Life & Group                                                  |           | (79)            | (45)    | (50) |   |
| Loss Portfolio Transfer                                                                                                                                                          |           |            |    |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                              |           |            |    |   |   |
| 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 A&EP adverse development before consideration of LPT                                                                                                                     |           | 60              | 200     | $ 150 |   |
| Cumulative amounts ceded under AEP Loss Portfolio Transfer                                                                                                                       |           | 2,900           | 2,800   |   |   |
| Deferred reinsurance benefit yet to be recognized                                                                                                                                |           | 326             | 334     |   |   |
| Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer                                                                                |           | 3,100           | $ 2,800 |   |   |
| Hurricane Harvey, Irma and Maria                                                                                                                                                 |           |            |    |   |   |
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                              |           |            |    |   |   |
| Catastrophe losses net of reinsurance                                                                                                                                            | $ 188          | $ 256           |    |   |   |

| 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 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
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
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                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
Cash payments made for reinsurance during the current period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585
+ Details**Name:**us-gaap_PaymentsForReinsurance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementScenarioAxis=cna_LossPortfolioTransferMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=cna_HurricaneHarveyIrmaandMariaMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |