| **Claim and Claim Adjustment Expense Reserves Net prior year claim and allocated claim adjustment expense reserve development for International segment (Details) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group                                                  | $ (175)        | $ (26)         | $ (187) | $ 23 |
| International [Member]                                                                                                                                                                      |           |           |   |   |
| Medical Professional Liability                                                                                                                                                              | (8)            | (3)            | (8) | (2) |
| Other Professional Liability                                                                                                                                                                | (11)           | 1              | (16) | (14) |
| Liability                                                                                                                                                                                   | (5)            | (3)            | (12) | (9) |
| Property & Marine                                                                                                                                                                       | (5)            | (11)           | (19) | (10) |
| Other                                                                                                                                                                                       | (5)            | (1)            | 9 | (7) |
| Commutations                                                                                                                                                                                | 0              | 0              | 0 | 10 |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group                                                  | $ (34)         | $ (17)         | $ (46) | $ (32) |

| X                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Medical Professional Liability
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentMedicalProfessionalLiability**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Other
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentOther**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Commutations
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentCommutations**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Liability
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentLiability**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Other Professional Liability
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentOtherProfessionalLiability**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Property & Marine
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentPropertyMarine**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_InternationalMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |