| **Reinsurance (Narrative) (Details) (USD $)**                                                                                        | 12 Months Ended                                                                                                                                                                                     |
|--------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| Funds held under reinsurance agreements, liability                                                                                   | $ 3,400,000,000us-gaap_FundsHeldUnderReinsuranceAgreementsLiability                                                                                                                                 | $ 3,900,000,000us-gaap_FundsHeldUnderReinsuranceAgreementsLiability |   |
| Reinsurance recoveries reported in insurance claims and policyholders' benefits reported on the Consolidated Statement of Operations | 1,379,000,000us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded                                                                                                                        | 1,450,000,000us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded | 1,443,000,000us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded |
| Subsidiaries of Berkshire Hathaway Group [Member]                                                                                    |                                                                                                                                                                                                |   |   |
| Largest Recoverables From Single Reinsurer                                                                                           | 2,565,000,000cna_LargestRecoverablesFromSingleReinsurer/ us-gaap_CededCreditRiskAxis= cna_SubsidiariesOfBerkshireHathawayGroupMember                                                                |   |   |
| Subsidiaries of Hartford Insurance Group [Member]                                                                                    |                                                                                                                                                                                                |   |   |
| Largest Recoverables From Single Reinsurer                                                                                           | 244,000,000cna_LargestRecoverablesFromSingleReinsurer/ us-gaap_CededCreditRiskAxis= cna_SubsidiariesOfHartfordInsuranceGroupMember                                                                  |   |   |
| Subsidiaries of Wilton Re [Member]                                                                                                   |                                                                                                                                                                                                |   |   |
| Largest Recoverables From Single Reinsurer                                                                                           | 185,000,000cna_LargestRecoverablesFromSingleReinsurer/ us-gaap_CededCreditRiskAxis= cna_SubsidiariesofWiltonReGroupMember                                                                           |   |   |
| Significant Captive Program [Member]                                                                                                 |                                                                                                                                                                                                |   |   |
| Direct and ceded earned premiums                                                                                                     | 2,643,000,000cna_DirectAndCededEarnedPremiumsRelatedToBusinessThatIsOneHundredPercentReinsuredAsResultOfSignificantCaptiveProgram/ us-gaap_CededCreditRiskAxis= cna_SignificantCaptiveProgramMember | 2,156,000,000cna_DirectAndCededEarnedPremiumsRelatedToBusinessThatIsOneHundredPercentReinsuredAsResultOfSignificantCaptiveProgram/ us-gaap_CededCreditRiskAxis= cna_SignificantCaptiveProgramMember | 1,794,000,000cna_DirectAndCededEarnedPremiumsRelatedToBusinessThatIsOneHundredPercentReinsuredAsResultOfSignificantCaptiveProgram/ us-gaap_CededCreditRiskAxis= cna_SignificantCaptiveProgramMember |
| Reinsurance recoveries reported in insurance claims and policyholders' benefits reported on the Consolidated Statement of Operations | $ 1,458,000,000us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded/ us-gaap_CededCreditRiskAxis= cna_SignificantCaptiveProgramMember                                                    | $ 712,000,000us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded/ us-gaap_CededCreditRiskAxis= cna_SignificantCaptiveProgramMember | $ 814,000,000us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded/ us-gaap_CededCreditRiskAxis= cna_SignificantCaptiveProgramMember |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Direct and ceded earned premiums related to business that is one hundred percent reinsured as a result of a significant captive program
+ References
No definition available.
+ Details**Name:**cna_DirectAndCededEarnedPremiumsRelatedToBusinessThatIsOneHundredPercentReinsuredAsResultOfSignificantCaptiveProgram**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Largest Recoverables from Single Reinsurer
+ References
No definition available.
+ Details**Name:**cna_LargestRecoverablesFromSingleReinsurer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of funds held as security for payments to be received under reinsurance contracts.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.13(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph c -Article 7
+ Details**Name:**us-gaap_FundsHeldUnderReinsuranceAgreementsLiability**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of reduction of provision for policy benefits and costs incurred for policies ceded.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387
+ Details**Name:**us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CededCreditRiskAxis=cna_SignificantCaptiveProgramMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CededCreditRiskAxis=cna_SubsidiariesOfBerkshireHathawayGroupMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CededCreditRiskAxis=cna_SubsidiariesOfHartfordInsuranceGroupMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CededCreditRiskAxis=cna_SubsidiariesofWiltonReGroupMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |