| **Schedule II. Condensed Financial Information of Registrant (Parent Company) (Narrative) (Details)** | Dec. 31, 2019 |
|-------------------------------------------------------------------------------------------------------|---------------|
| CNAF Consolidated | Loews                                                                             |          |
| **Condensed Financial Statements, Captions [Line Items]**                                             |          |
| Noncontrolling interest, ownership percentage by parent                                               | 89.00%        |

| 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 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_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:** |