| **Investments (Narrative) (Details) (USD $)In Millions, unless otherwise specified**        | Mar. 31, 2015                                                                             | Dec. 31, 2014                                                                             |
|---------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------|
| **Investments [Abstract]**                                                                  |                                                                                      |                                                                                      |
| Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments | $ 1,370cna_ReductionOfNetUnrealizedGainsOnInvestmentsIncludedInAociDueToShadowAdjustments | $ 1,288cna_ReductionOfNetUnrealizedGainsOnInvestmentsIncludedInAociDueToShadowAdjustments |
| Future capital call commitments for limited partnership investments                         | 338cna_FutureCapitalCallCommitments                                                       |                                                                                      |
| Mortgage loan commitments                                                                   | 41cna_MortgageLoanCommitments                                                             |                                                                                      |
| Commitments to purchase or fund privately placed debt securities                            | 82cna_CommitmentsToPurchaseOrFundAdditionalAmounts                                        |                                                                                      |
| Commitments to sell various privately placed debt securities                                | $ 71cna_CommitmentsToSellVariousPrivatelyPlacedDebtSecurities                             |                                                                                      |

| 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                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |