| **Reinsurance (Components of Written Premiums) (Details) - USD ($) $ in Millions**                     | 12 Months Ended |
|--------------------------------------------------------------------------------------------------------|-----------------|
||
| **Effects of Reinsurance [Line Items]**                                                                |            |   |   |
| Direct premiums written                                                                                | $ 11,141        | $ 10,946 | $ 10,345 |
| Assumed premiums written                                                                               | 377             | 297 | 319 |
| Ceded premiums written                                                                                 | 4,449           | 4,255 | 3,702 |
| Net written premiums                                                                                   | 7,069           | 6,988 | 6,962 |
| Property and Casualty Insurance                                                                        |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                                |            |   |   |
| Direct premiums written                                                                                | 10,655          | 10,451 | 9,852 |
| Assumed premiums written                                                                               | 327             | 245 | 270 |
| Ceded premiums written                                                                                 | 4,449           | 4,255 | 3,702 |
| Net written premiums                                                                                   | 6,533           | 6,441 | 6,420 |
| Long term care Insurance Product Line                                                                  |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                                |            |   |   |
| Direct premiums written                                                                                | 486             | 495 | 493 |
| Assumed premiums written                                                                               | 50              | 52 | 49 |
| Ceded premiums written                                                                                 | 0               | 0 | 0 |
| Net written premiums                                                                                   | $ 536           | $ 547 | $ 542 |
| Percentage of assumed premiums written to net premiums written                                         |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                                |            |   |   |
| Percentage of assumed premium written to premium written net                                           | 5.30%           | 4.30% | 4.60% |
| Percentage of assumed premiums written to net premiums written | Property and Casualty Insurance       |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                                |            |   |   |
| Percentage of assumed premium written to premium written net                                           | 5.00%           | 3.80% | 4.20% |
| Percentage of assumed premiums written to net premiums written | Long term care Insurance Product Line |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                                |            |   |   |
| Percentage of assumed premium written to premium written net                                           | 9.30%           | 9.50% | 9.00% |

| 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
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| 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 (c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -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 FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b) -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
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_EffectsOfReinsuranceLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**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 (c) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b) -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:**us-gaap_ProductOrServiceAxis=us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ProductOrServiceAxis=cna_LongtermcareInsuranceProductLineMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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