| **Schedule II. Condensed Financial Information of Registrant (Parent Company) (Narrative) (Details) - USD ($) $ in Millions** | Dec. 31, 2018 | Dec. 31, 2017 |
|-------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Parent Company                                                                                                                |          |          |
| **Condensed Financial Statements, Captions [Line Items]**                                                                     |          |          |
| Guarantor obligations, current carrying calue                                                                                 | $ 5           | $ 5           |
| Guarantor obligations, maximum exposure, undiscounted                                                                         | $ 1,800       |          |
| CNAF Consolidated | Loews                                                                                                     |          |          |
| **Condensed Financial Statements, Captions [Line Items]**                                                                     |          |          |
| Noncontrolling interest, ownership percentage by parent                                                                       | 89.00%        |          |
| Indemnification Agreement                                                                                                     |          |          |
| **Condensed Financial Statements, Captions [Line Items]**                                                                     |          |          |
| Guarantor obligations, maximum exposure, undiscounted                                                                         | $ 252         |          |
| Indemnification Agreement | Parent Company                                                                                    |          |          |
| **Condensed Financial Statements, Captions [Line Items]**                                                                     |          |          |
| Guarantor obligations, maximum exposure, undiscounted                                                                         | $ 625         |          |

| 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:**srt_CondensedFinancialStatementsCaptionsLineItems**Namespace Prefix:**srt_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248
+ Details**Name:**us-gaap_GuaranteeObligationsCurrentCarryingValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248
+ Details**Name:**us-gaap_GuaranteeObligationsMaximumExposure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| 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                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ConsolidatedEntitiesAxis=srt_ParentCompanyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| 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                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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