| **Reinsurance (Components of Written Premiums) (Details) - USD ($) $ in Millions**                                 | 12 Months Ended |
|--------------------------------------------------------------------------------------------------------------------|-----------------|
||
| Direct premiums written                                                                                            | $ 10,345        | $ 9,787 | $ 9,608 |
| Assumed premiums written                                                                                           | 319             | 325 | 296 |
| Ceded premiums written                                                                                             | 3,702           | 3,024 | 2,556 |
| Net written premiums                                                                                               | 6,962           | 7,088 | 7,348 |
| Property and Casualty Insurance [Member]                                                                           |            |   |   |
| Direct premiums written                                                                                            | 9,852           | 9,283 | 9,103 |
| Assumed premiums written                                                                                           | 270             | 276 | 249 |
| Ceded premiums written                                                                                             | 3,702           | 3,024 | 2,556 |
| Net written premiums                                                                                               | 6,420           | 6,535 | 6,796 |
| Accident and Health Insurance [Member]                                                                             |            |   |   |
| Direct premiums written                                                                                            | 493             | 504 | 505 |
| Assumed premiums written                                                                                           | 49              | 49 | 47 |
| Ceded premiums written                                                                                             | 0               | 0 | 0 |
| Net written premiums                                                                                               | $ 542           | $ 553 | $ 552 |
| Percentage of assumed premiums written to net premiums written [Member]                                            |            |   |   |
| Percentage of assumed premium written to premium written net                                                       | 4.60%           | 4.60% | 4.00% |
| 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%           | 4.20% | 3.70% |
| Percentage of assumed premiums written to net premiums written [Member] | Accident and Health Insurance [Member]   |            |   |   |
| Percentage of assumed premium written to premium written net                                                       | 9.00%           | 8.90% | 8.50% |

| 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 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=d3e27261-158547Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12
+ 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 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=d3e27261-158547Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12
+ 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 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=d3e27261-158547Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12
+ Details**Name:**us-gaap_PremiumsWrittenNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis=us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember**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:**cna_PercentageofassumedpremiumswrittentonetpremiumswrittenAxis=cna_PercentageOfAssumedPremiumsWrittenToNetPremiumsWrittenMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |