| **Claim and Claim Adjustment Expense Reserves Impact of Loss Portfolio Transfer on the Consolidated Statement of Operations (Details) - Loss Portfolio Transfer [Member] - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended | 48 Months Ended |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|-----------------|
||
| Net AEP adverse development before consideration of LPT                                                                                                                                            | $ 0            | $ 200          | $ 0             | $ 200 | $ 150 | $ 900 |
| Provision for uncollectible third-party reinsurance on AEP                                                                                                                                         | 0              |           | 0               | 0 | 0 |   |
| Additional amounts ceded under LPT                                                                                                                                                                 | 0              |           | 0               | 200 | 150 |   |
| Retroactive reinsurance benefit recognized                                                                                                                                                         | (12)           |           | (4)             | (94) | (75) |   |
| Pretax impact of deferred retroactive reinsurance benefit                                                                                                                                          | $ (12)         |           | $ (4)           | $ 106 | $ 75 |   |

| X                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Additional amounts ceded under LPT
+ References
No definition available.
+ Details**Name:**cna_AdditionalamountscededunderLPT**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**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
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 on AEP
+ References
No definition available.
+ Details**Name:**cna_ProvisionforuncollectiblethirdpartyreinsuranceonAEP**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:**us-gaap_StatementScenarioAxis=cna_LossPortfolioTransferMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |