| **Reinsurance (Components of Written Premiums) (Details) - USD ($) $ in Millions**                     | 12 Months Ended |
|--------------------------------------------------------------------------------------------------------|-----------------|
||
| **Effects of Reinsurance [Line Items]**                                                                |            |   |   |
| Direct premiums written                                                                                | $ 11,894        | $ 11,568 | $ 11,141 |
| Assumed premiums written                                                                               | 331             | 360 | 377 |
| Ceded premiums written                                                                                 | 4,569           | 4,583 | 4,449 |
| Net written premiums                                                                                   | 7,656           | 7,345 | 7,069 |
| Property and Casualty Insurance                                                                        |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                                |            |   |   |
| Direct premiums written                                                                                | 11,421          | 11,094 | 10,655 |
| Assumed premiums written                                                                               | 281             | 310 | 327 |
| Ceded premiums written                                                                                 | 4,569           | 4,583 | 4,449 |
| Net written premiums                                                                                   | 7,133           | 6,821 | 6,533 |
| Long term care Insurance Product Line                                                                  |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                                |            |   |   |
| Direct premiums written                                                                                | 473             | 474 | 486 |
| Assumed premiums written                                                                               | 50              | 50 | 50 |
| Ceded premiums written                                                                                 | 0               | 0 | 0 |
| Net written premiums                                                                                   | $ 523           | $ 524 | $ 536 |
| Percentage of assumed premiums written to net premiums written                                         |            |   |   |
| **Effects of Reinsurance [Line Items]**                                                                |            |   |   |
| Percentage of assumed premium written to premium written net                                           | 4.30%           | 4.90% | 5.30% |
| 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                                           | 3.90%           | 4.50% | 5.00% |
| 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                                           | 9.60%           | 9.50% | 9.30% |

| X                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Assumed Premium Written to Premium Written Net
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of written premiums assumed from other entities.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(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://fasb.org/us-gaap/role/ref/legacyRef -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=d3e27261-158547Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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=d3e27261-158547
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| 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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547
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| 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/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -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=d3e27261-158547
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| X                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cna_LongtermcareInsuranceProductLineMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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