| **Claim and Claim Adjustment Expense Reserves (Impact of Loss Portfolio Transfer on the Consolidated Statement of Operations) (Details) - Asbestos and Environmental Reserves - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Additional amounts ceded under LPT:**                                                                                                                                                                 |           |           |   |   |
| Net A&EP adverse development before consideration of LPT                                                                                                                                            | $ 0            | $ 0            | $ 113 | $ 60 |
| Provision for uncollectible third-party reinsurance on A&EP                                                                                                                                         | 0              | 0              | (16) | 0 |
| Total additional amounts ceded under LPT                                                                                                                                                                | 0              | 0              | 97 | 60 |
| Retroactive reinsurance benefit recognized                                                                                                                                                              | (12)           | (17)           | (84) | (60) |
| Pretax impact of deferred retroactive reinsurance                                                                                                                                                       | $ (12)         | $ (17)         | $ 13 | $ 0 |

| X                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Additional Amounts Ceded[Abstract]
+ References
No definition available.
+ Details**Name:**cna_AdditionalAmountsCededAbstract**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net AEP adverse development before consideration of LPT
+ References
No definition available.
+ Details**Name:**cna_NetAEPadversedevelopmentbeforeconsiderationofLPT**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Policyholder Benefits and Claims Incurred, Additional Amount Ceded
+ References
No definition available.
+ Details**Name:**cna_PolicyholderBenefitsandClaimsIncurredAdditionalAmountCeded**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax impact of deferred retroactive reinsurance benefit
+ References
No definition available.
+ Details**Name:**cna_Pretaximpactofdeferredretroactivereinsurancebenefit**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Provision For Uncollectible Third-Party Reinsurance
+ References
No definition available.
+ Details**Name:**cna_ProvisionForUncollectibleThirdPartyReinsurance**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Retroactive reinsurance benefit recognized
+ References
No definition available.
+ Details**Name:**cna_Retroactivereinsurancebenefitrecognized**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cna_AsbestosandEnvironmentalReservesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |