| **Reinsurance (Components of Written Premiums) (Details) (USD $)In Millions, unless otherwise specified**          | 12 Months Ended                                                                                                                                                                                                                                                                                                               |
|--------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| Direct premiums written                                                                                            | $ 9,787us-gaap_DirectPremiumsWritten                                                                                                                                                                                                                                                                                          | $ 9,608us-gaap_DirectPremiumsWritten | $ 8,973us-gaap_DirectPremiumsWritten |
| Assumed premiums written                                                                                           | 325us-gaap_AssumedPremiumsWritten                                                                                                                                                                                                                                                                                             | 296us-gaap_AssumedPremiumsWritten | 216us-gaap_AssumedPremiumsWritten |
| Ceded premiums written                                                                                             | 3,024us-gaap_CededPremiumsWritten                                                                                                                                                                                                                                                                                             | 2,556us-gaap_CededPremiumsWritten | 2,225us-gaap_CededPremiumsWritten |
| Net written premiums                                                                                               | 7,088us-gaap_PremiumsWrittenNet                                                                                                                                                                                                                                                                                               | 7,348us-gaap_PremiumsWrittenNet | 6,964us-gaap_PremiumsWrittenNet |
| Property and Casualty Insurance [Member]                                                                           |                                                                                                                                                                                                                                                                                                                          |   |   |
| Direct premiums written                                                                                            | 9,283us-gaap_DirectPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember                                                                                                                                                          | 9,103us-gaap_DirectPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember | 8,467us-gaap_DirectPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember |
| Assumed premiums written                                                                                           | 276us-gaap_AssumedPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember                                                                                                                                                           | 249us-gaap_AssumedPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember | 169us-gaap_AssumedPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember |
| Ceded premiums written                                                                                             | 3,024us-gaap_CededPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember                                                                                                                                                           | 2,556us-gaap_CededPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember | 2,225us-gaap_CededPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember |
| Net written premiums                                                                                               | 6,535us-gaap_PremiumsWrittenNet/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember                                                                                                                                                             | 6,796us-gaap_PremiumsWrittenNet/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember | 6,411us-gaap_PremiumsWrittenNet/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember |
| Accident and Health Insurance [Member]                                                                             |                                                                                                                                                                                                                                                                                                                          |   |   |
| Direct premiums written                                                                                            | 504us-gaap_DirectPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember                                                                                                                                                                       | 505us-gaap_DirectPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember | 506us-gaap_DirectPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember |
| Assumed premiums written                                                                                           | 49us-gaap_AssumedPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember                                                                                                                                                                       | 47us-gaap_AssumedPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember | 47us-gaap_AssumedPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember |
| Ceded premiums written                                                                                             | 0us-gaap_CededPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember                                                                                                                                                                          | 0us-gaap_CededPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember | 0us-gaap_CededPremiumsWritten/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember |
| Net written premiums                                                                                               | $ 553us-gaap_PremiumsWrittenNet/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember                                                                                                                                                                        | $ 552us-gaap_PremiumsWrittenNet/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember | $ 553us-gaap_PremiumsWrittenNet/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember |
| Percentage of assumed premiums written to net premiums written [Member]                                            |                                                                                                                                                                                                                                                                                                                          |   |   |
| Percentage of assumed premium written to premium written net                                                       | 4.60%cna_PercentageOfAssumedPremiumWrittenToPremiumWrittenNet/ cna_PercentageOfWrittenAmountAssumedToNetWrittenEarnedAxis= cna_PercentageOfAssumedPremiumsWrittenToNetPremiumsWrittenMember                                                                                                                                   | 4.00%cna_PercentageOfAssumedPremiumWrittenToPremiumWrittenNet/ cna_PercentageOfWrittenAmountAssumedToNetWrittenEarnedAxis= cna_PercentageOfAssumedPremiumsWrittenToNetPremiumsWrittenMember | 3.10%cna_PercentageOfAssumedPremiumWrittenToPremiumWrittenNet/ cna_PercentageOfWrittenAmountAssumedToNetWrittenEarnedAxis= cna_PercentageOfAssumedPremiumsWrittenToNetPremiumsWrittenMember |
| Percentage of assumed premiums written to net premiums written [Member] | Property and Casualty Insurance [Member] |                                                                                                                                                                                                                                                                                                                          |   |   |
| Percentage of assumed premium written to premium written net                                                       | 4.20%cna_PercentageOfAssumedPremiumWrittenToPremiumWrittenNet/ cna_PercentageOfWrittenAmountAssumedToNetWrittenEarnedAxis= cna_PercentageOfAssumedPremiumsWrittenToNetPremiumsWrittenMember/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember | 3.70%cna_PercentageOfAssumedPremiumWrittenToPremiumWrittenNet/ cna_PercentageOfWrittenAmountAssumedToNetWrittenEarnedAxis= cna_PercentageOfAssumedPremiumsWrittenToNetPremiumsWrittenMember/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember | 2.60%cna_PercentageOfAssumedPremiumWrittenToPremiumWrittenNet/ cna_PercentageOfWrittenAmountAssumedToNetWrittenEarnedAxis= cna_PercentageOfAssumedPremiumsWrittenToNetPremiumsWrittenMember/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember |
| Percentage of assumed premiums written to net premiums written [Member] | Accident and Health Insurance [Member]   |                                                                                                                                                                                                                                                                                                                          |   |   |
| Percentage of assumed premium written to premium written net                                                       | 8.90%cna_PercentageOfAssumedPremiumWrittenToPremiumWrittenNet/ cna_PercentageOfWrittenAmountAssumedToNetWrittenEarnedAxis= cna_PercentageOfAssumedPremiumsWrittenToNetPremiumsWrittenMember/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember            | 8.50%cna_PercentageOfAssumedPremiumWrittenToPremiumWrittenNet/ cna_PercentageOfWrittenAmountAssumedToNetWrittenEarnedAxis= cna_PercentageOfAssumedPremiumsWrittenToNetPremiumsWrittenMember/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember | 8.50%cna_PercentageOfAssumedPremiumWrittenToPremiumWrittenNet/ cna_PercentageOfWrittenAmountAssumedToNetWrittenEarnedAxis= cna_PercentageOfAssumedPremiumsWrittenToNetPremiumsWrittenMember/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis= us-gaap_AccidentAndHealthInsuranceSegmentMember |

| X                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Assumed Premium Written to Premium Written Net
+ References
No definition available.
+ Details**Name:**cna_PercentageOfAssumedPremiumWrittenToPremiumWrittenNet**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of written premiums assumed from other entities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(2),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547
+ Details**Name:**us-gaap_AssumedPremiumsWritten**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of written premiums ceded to other entities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547
+ Details**Name:**us-gaap_CededPremiumsWritten**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547
+ Details**Name:**us-gaap_DirectPremiumsWritten**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547
+ Details**Name:**us-gaap_PremiumsWrittenNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cna_PercentageOfWrittenAmountAssumedToNetWrittenEarnedAxis=cna_PercentageOfAssumedPremiumsWrittenToNetPremiumsWrittenMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=us-gaap_AccidentAndHealthInsuranceSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |