| **Reinsurance (Components of Written Premiums) (Details) - USD ($) $ in Millions**     | 12 Months Ended |
|----------------------------------------------------------------------------------------|-----------------|
||
| **Premiums Written, Net [Abstract]**                                                   |            |   |   |
| Direct                                                                                 | $ 14,264        | $ 13,587 | $ 12,612 |
| Assumed                                                                                | 281             | 303 | 279 |
| Ceded                                                                                  | 5,417           | 5,485 | 4,832 |
| Net                                                                                    | 9,128           | 8,405 | 8,059 |
| Property and casualty                                                                  |            |   |   |
| **Premiums Written, Net [Abstract]**                                                   |            |   |   |
| Direct                                                                                 | 13,843          | 13,150 | 12,168 |
| Assumed                                                                                | 235             | 255 | 229 |
| Ceded                                                                                  | 5,417           | 5,485 | 4,832 |
| Net                                                                                    | 8,661           | 7,920 | 7,565 |
| Long term care                                                                         |            |   |   |
| **Premiums Written, Net [Abstract]**                                                   |            |   |   |
| Direct                                                                                 | 421             | 437 | 444 |
| Assumed                                                                                | 46              | 48 | 50 |
| Ceded                                                                                  | 0               | 0 | 0 |
| Net                                                                                    | $ 467           | $ 485 | $ 494 |
| Percentage of assumed premiums written to net premiums written                         |            |   |   |
| **Premiums Written, Net [Abstract]**                                                   |            |   |   |
| Assumed/ Net %                                                                         | 3.10%           | 3.60% | 3.50% |
| Percentage of assumed premiums written to net premiums written | Property and casualty |            |   |   |
| **Premiums Written, Net [Abstract]**                                                   |            |   |   |
| Assumed/ Net %                                                                         | 2.70%           | 3.20% | 3.00% |
| Percentage of assumed premiums written to net premiums written | Long term care        |            |   |   |
| **Premiums Written, Net [Abstract]**                                                   |            |   |   |
| Assumed/ Net %                                                                         | 9.90%           | 9.90% | 10.10% |

| X                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Assumed Premium Written to Premium Written Net
+ References
No definition available.
+ Details**Name:**cna_PercentageOfAssumedPremiumWrittenToPremiumWrittenNet**Namespace Prefix:**cna_**Data Type:**dtr-types: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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(2) -URI https://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://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 https://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 https://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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI https://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/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI https://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 https://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                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_PremiumsWrittenNetConsolidatedAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cna_LongtermcareInsuranceProductLineMember**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:** |