| **Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Specialty - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development) (Details) - Specialty - USD ($) $ in Millions** | 12 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Liability for Claims and Claims Adjustment Expense [Line Items]**                                                                                                                                                |            |   |   |
| Medical Professional Liability                                                                                                                                                                                     | $ 23            | $ 35 | $ 75 |
| Other Professional Liability and Management Liability                                                                                                                                                              | 24              | (15) | (69) |
| Surety                                                                                                                                                                                                             | (73)            | (69) | (92) |
| Warranty                                                                                                                                                                                                           | (14)            | (7) | (15) |
| Other                                                                                                                                                                                                              | (5)             | (5) | 9 |
| Total pretax (favorable) unfavorable development                                                                                                                                                                   | $ (45)          | $ (61) | $ (92) |

| 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, Surety
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentSurety**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, Warranty
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentWarranty**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 and Management Liability
+ References
No definition available.
+ Details**Name:**cna_NetprioryearclaimandallocatedclaimadjustmentexpensereservedevelopmentOtherProfessionalLiabilityandManagementLiability**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                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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_LiabilityForClaimsAndClaimsAdjustmentExpenseLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtySegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |