| **General (Narrative) (Details) - USD ($) $ in Millions**                        | Sep. 30, 2019 | Jan. 01, 2019 | Dec. 31, 2018 |
|----------------------------------------------------------------------------------|---------------|---------------|---------------|
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]** |          |          |          |
| Other assets                                                                     | $ (1,571)     | $ (1,445)     | $ (1,208)     |
| CNAF Consolidated | Loews                                                        |          |          |          |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]** |          |          |          |
| Noncontrolling interest, ownership percentage by parent                          | 89.00%        |          |          |
| Adjustments Due to Adoption of Topic 842                                         |          |          |          |
| **New Accounting Pronouncements or Change in Accounting Principle [Line Items]** |          |          |          |
| Other assets                                                                     | $ 75          | $ (237)       |          |

| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The parent entity's interest in net assets of the subsidiary, expressed as a percentage.
+ References
No definition available.
+ Details**Name:**us-gaap_MinorityInterestOwnershipPercentageByParent**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| 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_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of assets classified as other.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(17)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(10)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(10)) -URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910
+ Details**Name:**us-gaap_OtherAssets**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_OwnershipAxis=cna_CNAFConsolidatedMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**dei_LegalEntityAxis=cna_LoewsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201602Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |