| **Fair Value (Quantitative information about significant unobservable inputs in the fair value measurement of level 3 assets) (Details) - Income Approach Valuation Technique [Member] - USD ($) $ in Millions** | Sep. 30, 2016 | Dec. 31, 2015 |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Fixed maturity securities [Member]                                                                                                                                                                               |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                             |          |          |
| Assets, Fair Value Disclosure                                                                                                                                                                                    | $ 211         | $ 138         |
| Fixed maturity securities [Member] | Minimum [Member]                                                                                                                                                            |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                             |          |          |
| Credit Spread                                                                                                                                                                                                    | 2.00%         | 3.00%         |
| Fixed maturity securities [Member] | Maximum [Member]                                                                                                                                                            |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                             |          |          |
| Credit Spread                                                                                                                                                                                                    | 40.00%        | 184.00%       |
| Fixed maturity securities [Member] | Weighted Average [Member]                                                                                                                                                   |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                             |          |          |
| Credit Spread                                                                                                                                                                                                    | 6.00%         | 6.00%         |
| Life settlement contracts, included in Other assets [Member]                                                                                                                                                     |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                             |          |          |
| Assets, Fair Value Disclosure                                                                                                                                                                                    | $ 67          | $ 74          |
| Life settlement contracts, included in Other assets [Member] | Minimum [Member]                                                                                                                                  |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                             |          |          |
| Mortality assumption                                                                                                                                                                                             | 55.00%        | 55.00%        |
| Life settlement contracts, included in Other assets [Member] | Maximum [Member]                                                                                                                                  |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                             |          |          |
| Mortality assumption                                                                                                                                                                                             | 1676.00%      | 1676.00%      |
| Life settlement contracts, included in Other assets [Member] | Weighted Average [Member]                                                                                                                         |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                             |          |          |
| Mortality assumption                                                                                                                                                                                             | 162.00%       | 164.00%       |
| Life settlement contracts, included in Other assets [Member] | Rate [Member]                                                                                                                                     |          |          |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                             |          |          |
| Discount rate risk premium                                                                                                                                                                                       | 9.00%         | 9.00%         |

| X                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Discount rate risk premium
+ References
No definition available.
+ Details**Name:**cna_DiscountRateRiskPremium**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
fair value inputs, credit risk, counterparty
+ References
No definition available.
+ Details**Name:**cna_Fairvalueinputscreditriskcounterparty**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Mortality assumption
+ References
No definition available.
+ Details**Name:**cna_MortalityAssumption**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
+ Details**Name:**us-gaap_AssetsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_FairValueInputsAssetsQuantitativeInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=us-gaap_FixedIncomeSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ValuationTechniqueAxis=us-gaap_IncomeApproachValuationTechniqueMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MinimumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MaximumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_WeightedAverageMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=cna_LifeSettlementContractsIncludedInOtherAssetsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=cna_RateMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |