| **Business Segments (Revenues by Line of Business) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended                                                                                        |
|---------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------|
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| Revenues                                                                                                      | $ 2,352us-gaap_Revenues                                                                               | $ 2,463us-gaap_Revenues |
| Specialty [Domain]                                                                                            |                                                                                                  |   |
| Revenues                                                                                                      | 917us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_SpecialtyDomain                       | 915us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_SpecialtyDomain |
| Management & Professional Liability [Member]                                                              |                                                                                                  |   |
| Revenues                                                                                                      | 697us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_ProfessionalManagementLiabilityMember | 704us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_ProfessionalManagementLiabilityMember |
| Surety [Member]                                                                                               |                                                                                                  |   |
| Revenues                                                                                                      | 120us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_SuretyMember                          | 122us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_SuretyMember |
| Warranty & Alternative Risks [Member]                                                                     |                                                                                                  |   |
| Revenues                                                                                                      | 100us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_WarrantyAlternativeRisksMember        | 89us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_WarrantyAlternativeRisksMember |
| Commercial [Domain]                                                                                           |                                                                                                  |   |
| Revenues                                                                                                      | 895us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_CommercialDomain                      | 946us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_CommercialDomain |
| Middle Markets [Member]                                                                                       |                                                                                                  |   |
| Revenues                                                                                                      | 409us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_MiddleMarketsMember                   | 403us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_MiddleMarketsMember |
| Small Business [Member]                                                                                       |                                                                                                  |   |
| Revenues                                                                                                      | 165us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_SmallBusinessMember                   | 184us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_SmallBusinessMember |
| Other Commercial Insurance [Member]                                                                           |                                                                                                  |   |
| Revenues                                                                                                      | 321us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_OtherCommercialInsuranceMember        | 359us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_OtherCommercialInsuranceMember |
| International [Domain]                                                                                        |                                                                                                  |   |
| Revenues                                                                                                      | 206us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_InternationalDomain                   | 259us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_InternationalDomain |
| Canada [Member]                                                                                               |                                                                                                  |   |
| Revenues                                                                                                      | 55us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_CanadaMember                           | 71us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_CanadaMember |
| CNA Europe [Member]                                                                                           |                                                                                                  |   |
| Revenues                                                                                                      | 77us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_CNAEuropeMember                        | 88us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_CNAEuropeMember |
| Hardy [Member]                                                                                                |                                                                                                  |   |
| Revenues                                                                                                      | 74us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_HardyMember                            | 100us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_HardyMember |
| Life & Group Non-Core [Domain]                                                                            |                                                                                                  |   |
| Revenues                                                                                                      | 327us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_LifeGroupNonCoreDomain                | 331us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_LifeGroupNonCoreDomain |
| Corporate and Other [Domain]                                                                                  |                                                                                                  |   |
| Revenues                                                                                                      | 8us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_CorporateandOtherDomain                 | 13us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_CorporateandOtherDomain |
| Intersegment Eliminations [Domain]                                                                            |                                                                                                  |   |
| Revenues                                                                                                      | $ (1)us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_IntersegmentEliminationsDomain      | $ (1)us-gaap_Revenues/ us-gaap_StatementBusinessSegmentsAxis= cna_IntersegmentEliminationsDomain |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Article 5
+ Details**Name:**us-gaap_Revenues**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CanadaMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CNAEuropeMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CommercialDomain**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CorporateandOtherDomain**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_HardyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_InternationalDomain**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_IntersegmentEliminationsDomain**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_LifeGroupNonCoreDomain**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_MiddleMarketsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_OtherCommercialInsuranceMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_ProfessionalManagementLiabilityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SmallBusinessMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtyDomain**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SuretyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_WarrantyAlternativeRisksMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |