| **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, unless otherwise specified** | 12 Months Ended                                                                                                                           |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------|
||
| Loss Portfolio Transfer [Member]                                                                                                                                                                                           |                                                                                                                                      |   |   |
| Net AEP adverse development before consideration of LPT                                                                                                                                                                    |                                                                                                                                      | $ 363cna_NetAEPadversedevelopmentbeforeconsiderationofLPT/ us-gaap_StatementScenarioAxis= cna_LossPortfolioTransferMember | $ 261cna_NetAEPadversedevelopmentbeforeconsiderationofLPT/ us-gaap_StatementScenarioAxis= cna_LossPortfolioTransferMember |
| Provision for uncollectible third-party reinsurance on AEP                                                                                                                                                                 |                                                                                                                                      | 140cna_ProvisionforuncollectiblethirdpartyreinsuranceonAEP/ us-gaap_StatementScenarioAxis= cna_LossPortfolioTransferMember |   |
| Additional amounts ceded under LPT                                                                                                                                                                                         |                                                                                                                                      | 503cna_AdditionalamountscededunderLPT/ us-gaap_StatementScenarioAxis= cna_LossPortfolioTransferMember | 261cna_AdditionalamountscededunderLPT/ us-gaap_StatementScenarioAxis= cna_LossPortfolioTransferMember |
| Retroactive reinsurance benefit recognized                                                                                                                                                                                 | (13)cna_Retroactivereinsurancebenefitrecognized/ us-gaap_StatementScenarioAxis= cna_LossPortfolioTransferMember                           | (314)cna_Retroactivereinsurancebenefitrecognized/ us-gaap_StatementScenarioAxis= cna_LossPortfolioTransferMember | (261)cna_Retroactivereinsurancebenefitrecognized/ us-gaap_StatementScenarioAxis= cna_LossPortfolioTransferMember |
| Pretax impact of unrecognized deferred retroactive reinsurance benefit                                                                                                                                                     | $ (13)cna_Pretaximpactofunrecognizeddeferredretroactivereinsurancebenefit/ us-gaap_StatementScenarioAxis= cna_LossPortfolioTransferMember | $ 189cna_Pretaximpactofunrecognizeddeferredretroactivereinsurancebenefit/ us-gaap_StatementScenarioAxis= cna_LossPortfolioTransferMember |   |

| 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 unrecognized deferred retroactive reinsurance benefit
+ References
No definition available.
+ Details**Name:**cna_Pretaximpactofunrecognizeddeferredretroactivereinsurancebenefit**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 |