| **Investments (Narrative) (Details) - USD ($) $ in Millions**                               | Sep. 30, 2015 | Dec. 31, 2014 |
|---------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments [Abstract]**                                                                  |          |          |
| Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments | $ 1,046       | $ 1,288       |
| Future capital call commitments for limited partnership investments                         | 414           |          |
| Mortgage loan commitments                                                                   | 29            |          |
| Commitments to purchase or fund privately placed debt securities                            | 81            |          |
| Commitments to sell various privately placed debt securities                                | $ 43          |          |

| X                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Commitments To Purchase Or Fund Additional Amounts
+ References
No definition available.
+ Details**Name:**cna_CommitmentsToPurchaseOrFundAdditionalAmounts**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Commitments To Sell Various Privately Placed Debt Securities
+ References
No definition available.
+ Details**Name:**cna_CommitmentsToSellVariousPrivatelyPlacedDebtSecurities**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Future capital call commitments to limited partnerships for which we would receive an ownership interest.
+ References
No definition available.
+ Details**Name:**cna_FutureCapitalCallCommitments**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Mortgage loan commitments, representing signed loan applications received and accepted.
+ References
No definition available.
+ Details**Name:**cna_MortgageLoanCommitments**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments
+ References
No definition available.
+ Details**Name:**cna_ReductionOfNetUnrealizedGainsOnInvestmentsIncludedInAociDueToShadowAdjustments**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_InvestmentsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |