| **General - Narrative (Details) $ in Millions**                                                | 3 Months Ended |
|------------------------------------------------------------------------------------------------|----------------|
||
| **Noncontrolling Interest [Line Items]**                                                       |           |
| Employee service share-based compensation, tax benefit realized from exercise of stock options | $ 3            |
| CNAF Consolidated | Loews                                                                      |           |
| **Noncontrolling Interest [Line Items]**                                                       |           |
| Ownership percentage of outstanding common stock of CNAF                                       | 90.00%         |

| X                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount
+ References
No definition available.
+ Details**Name:**cna_EffectiveIncomeTaxRateReconciliationSharebasedCompensationExcessTaxBenefitAmount**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| 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_MinorityInterestLineItems**Namespace Prefix:**us-gaap_**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:**cna_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:** |