| **Accumulated Other Comprehensive Income (Loss) Narrative (Details) (USD $)In Millions, unless otherwise specified**                                                | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|
| Pretax net unrealized gains (losses) on available-for-sale securities with OTTI losses reclassified out of AOCI and recognized in earnings                          | $ (28)        | $ (83)        | $ (42)        |
| Tax (expense) benefit on pretax net unrealized gains (losses) on available-for-sale securities with OTTI losses reclassified out of AOCI and recognized in earnings | 10            | 29            | 15            |
| Pretax net unrealized gains (losses) on other available-for-sale securities reclassified out of AOCI and recognized in earnings                                     | 89            | 60            | 137           |
| Tax (expense) benefit on pretax net unrealized gains (losses) on other available-for-sale securities reclassified out of AOCI and recognized in earnings            | $ (31)        | $ (21)        | $ (48)        |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of pretax net unrealized gains (losses) on available-for-sale securities with OTTI losses reclassified out of AOCI and recognized in earnings
+ References
No definition available.
+ Details**Name:**cna_PretaxNetUnrealizedGainsLossesOnAvailableForSaleSecuritiesWithOttiLossesReclassifiedOutOfAociAndRecongnizedInEarnings**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of pretax net unrealized gains (losses) on other available-for-sale securities reclassified out of AOCI and recognized in earnings
+ References
No definition available.
+ Details**Name:**cna_PretaxNetUnrealizedGainsLossesOnOtherAvailableForSaleSecuritiesReclassifiedOutOfAociAndRecognizedInEarnings**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tax (expense) benefit on the pretax net unrealized gains (losses) on available-for-sale securities with OTTI losses reclassified out of AOCI and recognized in earnings
+ References
No definition available.
+ Details**Name:**cna_TaxExpenseBenefitOnPretaxNetUnrealizedGainsLossesOnAvailableForSaleSecuritiesWithOttiLossesReclassifiedOutOfAociAndRecognizedInEarnings**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tax (expense) benefit on pretax net unrealized gains (losses) on other available-for-sale securities reclassified out of AOCI and recognized in earnings
+ References
No definition available.
+ Details**Name:**cna_TaxExpenseBenefitOnPretaxNetUnrealizedGainsLossesOnOtherAvailableForSaleSecuritiesReclassifiedOutOfAociAndRecognizedInEarnings**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |