| **Reinsurance (Components of Life Insurance Inforce) (Details) (USD $)In Millions, unless otherwise specified** | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 |
|-----------------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|
| Life Insurance Inforce, Direct                                                                                  | $ 6,528       | $ 8,015       | $ 9,159       |
| Life Insurance Inforce, Assumed                                                                                 |     |     |     |
| Life Insurance Inforce, Ceded                                                                                   | 6,515         | 8,001         | 9,144         |
| Life Insurance Inforce, Net                                                                                     | $ 13          | $ 14          | $ 15          |

| X                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Life Insurance Inforce, Assumed
+ References
No definition available.
+ Details**Name:**cna_LifeInsuranceInforceAssumed**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Life Insurance Inforce, Ceded
+ References
No definition available.
+ Details**Name:**cna_LifeInsuranceInforceCeded**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Life Insurance Inforce, Direct
+ References
No definition available.
+ Details**Name:**cna_LifeInsuranceInforceDirect**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Life Insurance Inforce, Net
+ References
No definition available.
+ Details**Name:**cna_LifeInsuranceInforceNet**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |