| **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]**                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | $ (175)        | $ (26)         | $ (187) | $ 23 |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (5)            | 3              | (11) | (30) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (180)          | (23)           | (198) | (7) |
| Specialty [Member]                                                                                                                         |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (130)          | (79)           | (141) | (123) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (2)            | (4)            | (10) | (12) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (132)          | (83)           | (151) | (135) |
| Commercial [Member]                                                                                                                        |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (11)           | 71             | 0 | 179 |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (5)            | 0              | (17) | (24) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (16)           | 71             | (17) | 155 |
| International [Member]                                                                                                                     |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (34)           | (17)           | (46) | (32) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 2              | 7              | 16 | 6 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (32)           | (10)           | (30) | (26) |
| Corporate and Other [Member]                                                                                                               |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | 0              | (1)            | 0 | (1) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 0              | 0              | 0 | 0 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | $ 0            | $ (1)          | $ 0 | $ (1) |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group
+ References
No definition available.
+ Details**Name:**cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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
Total Pretax Favorable Unfavorable Net Prior Year Development, excluding Life & Group
+ References
No definition available.
+ Details**Name:**cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup**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:** |