| **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                                                                                                                                                                    | $ 363           | $ 261 | $ 84 |
| Provision for uncollectible third-party reinsurance on AEP                                                                                                                                                                 | 140             |    |    |
| Additional amounts ceded under LPT                                                                                                                                                                                         | 503             | 261 | 84 |
| Retroactive reinsurance benefit recognized                                                                                                                                                                                 | (314)           | (261) | (84) |
| Pretax impact of unrecognized deferred retroactive reinsurance benefit                                                                                                                                                     | $ 189           |    |    |

| 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 |