| **Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|----------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Segment Reporting Information [Line Items]**                                                                 |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                               | $ (62)         | $ (115)        | $ (160) | $ (227) |
| Specialty                                                                                                      |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                               | (53)           | (99)           | (127) | (134) |
| Commercial                                                                                                     |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                               | (5)            | (17)           | (27) | (94) |
| International                                                                                                  |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                               | (2)            | 1              | (4) | 1 |
| Corporate & Other                                                                                          |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |           |           |   |   |
| Total pretax (favorable) unfavorable development                                                               | $ (2)          | $ 0            | $ (2) | $ 0 |

| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax Favorable Unfavorable Premium Development, excluding Life & Group
+ References
No definition available.
+ Details**Name:**cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**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_SegmentReportingInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CommercialMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_InternationalMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateAndOtherMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |