| **Fair Value (Assets and liabilities measured at fair value) (Details) - USD ($) $ in Millions** | Sep. 30, 2018 | Dec. 31, 2017 |
|--------------------------------------------------------------------------------------------------|---------------|---------------|
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | $ 39,628      | $ 41,487      |
| Equity securities                                                                                | 795           |          |
| Equity securities                                                                                |          | 695           |
| Short term and other                                                                             | 1,183         | 1,354         |
| Assets                                                                                           | 41,606        | 43,536        |
| **Liabilities**                                                                                  |          |          |
| Other liabilities                                                                                | (6)           | 3             |
| Liabilities                                                                                      | (6)           | 3             |
| Corporate and other bonds                                                                        |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 19,631        | 19,374        |
| States, municipalities and political subdivisions                                                |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 11,145        | 14,027        |
| Asset-backed                                                                                     |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 8,852         | 8,086         |
| Common Stock                                                                                     |          |          |
| **Assets**                                                                                       |          |          |
| Equity securities                                                                                | 105           |          |
| Equity securities                                                                                |          | 27            |
| Preferred stock                                                                                  |          |          |
| **Assets**                                                                                       |          |          |
| Equity securities                                                                                | 690           |          |
| Equity securities                                                                                |          | 668           |
| Level 1                                                                                          |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 183           | 128           |
| Equity securities                                                                                | 152           |          |
| Equity securities                                                                                |          | 91            |
| Short term and other                                                                             | 252           | 396           |
| Assets                                                                                           | 587           | 615           |
| **Liabilities**                                                                                  |          |          |
| Other liabilities                                                                                | 0             | 0             |
| Liabilities                                                                                      | 0             | 0             |
| Level 1 | Corporate and other bonds                                                              |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 183           | 128           |
| Level 1 | States, municipalities and political subdivisions                                      |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 0             | 0             |
| Level 1 | Asset-backed                                                                           |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 0             | 0             |
| Level 1 | Common Stock                                                                           |          |          |
| **Assets**                                                                                       |          |          |
| Equity securities                                                                                | 101           |          |
| Equity securities                                                                                |          | 23            |
| Level 1 | Preferred stock                                                                        |          |          |
| **Assets**                                                                                       |          |          |
| Equity securities                                                                                | 51            |          |
| Equity securities                                                                                |          | 68            |
| Level 2                                                                                          |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 38,959        | 40,925        |
| Equity securities                                                                                | 625           |          |
| Equity securities                                                                                |          | 584           |
| Short term and other                                                                             | 931           | 958           |
| Assets                                                                                           | 40,515        | 42,467        |
| **Liabilities**                                                                                  |          |          |
| Other liabilities                                                                                | (6)           | 3             |
| Liabilities                                                                                      | (6)           | 3             |
| Level 2 | Corporate and other bonds                                                              |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 19,260        | 19,148        |
| Level 2 | States, municipalities and political subdivisions                                      |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 11,145        | 14,026        |
| Level 2 | Asset-backed                                                                           |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 8,554         | 7,751         |
| Level 2 | Common Stock                                                                           |          |          |
| **Assets**                                                                                       |          |          |
| Equity securities                                                                                | 0             |          |
| Equity securities                                                                                |          | 0             |
| Level 2 | Preferred stock                                                                        |          |          |
| **Assets**                                                                                       |          |          |
| Equity securities                                                                                | 625           |          |
| Equity securities                                                                                |          | 584           |
| Level 3                                                                                          |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 486           | 434           |
| Equity securities                                                                                | 18            |          |
| Equity securities                                                                                |          | 20            |
| Short term and other                                                                             | 0             | 0             |
| Assets                                                                                           | 504           | 454           |
| **Liabilities**                                                                                  |          |          |
| Other liabilities                                                                                | 0             | 0             |
| Liabilities                                                                                      | 0             | 0             |
| Level 3 | Corporate and other bonds                                                              |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 188           | 98            |
| Level 3 | States, municipalities and political subdivisions                                      |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 0             | 1             |
| Level 3 | Asset-backed                                                                           |          |          |
| **Assets**                                                                                       |          |          |
| Fixed maturity securities                                                                        | 298           | 335           |
| Level 3 | Common Stock                                                                           |          |          |
| **Assets**                                                                                       |          |          |
| Equity securities                                                                                | 4             |          |
| Equity securities                                                                                |          | 4             |
| Level 3 | Preferred stock                                                                        |          |          |
| **Assets**                                                                                       |          |          |
| Equity securities                                                                                | $ 14          |          |
| Equity securities                                                                                |          | $ 16          |

| X                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Short-term Investments and Other
+ References
No definition available.
+ Details**Name:**cna_ShorttermInvestmentsandOther**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258
+ Details**Name:**us-gaap_AssetsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of equity securities categorized neither as held-to-maturity nor as trading.
+ References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesEquitySecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt security measured at amortized cost (held-to-maturity), investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and investment in debt security measured at fair value with change in fair value recognized in net income (trading).
+ References
No definition available.
+ Details**Name:**us-gaap_DebtSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740
+ Details**Name:**us-gaap_EquitySecuritiesFvNi**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of financial and nonfinancial obligations.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258
+ Details**Name:**us-gaap_LiabilitiesFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of other liabilities.
+ References
No definition available.
+ Details**Name:**us-gaap_OtherLiabilitiesFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentTypeAxis=us-gaap_CorporateDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentTypeAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentTypeAxis=us-gaap_AssetBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentTypeAxis=us-gaap_CommonStockMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentTypeAxis=us-gaap_PreferredStockMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |