| **Fair Value (Carrying amount and estimated fair value of financial instrument assets and liabilities which are not measured at fair value) (Details) (USD $)In Millions, unless otherwise specified** | Sep. 30, 2012 | Dec. 31, 2011 |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financial Assets [Abstract]**                                                                                                                                                                        |          |          |
| Notes receivable for the issuance of common stock                                                                                                                                                      | $ 21          | $ 22          |
| Mortgage loans                                                                                                                                                                                         | 358           | 234           |
| **Financial Liabilities [Abstract]**                                                                                                                                                                   |          |          |
| Short term debt                                                                                                                                                                                        | 13            | 83            |
| Long term debt                                                                                                                                                                                         | 2,557         | 2,525         |
| Carrying Amount [Member]                                                                                                                                                                               |          |          |
| **Financial Assets [Abstract]**                                                                                                                                                                        |          |          |
| Notes receivable for the issuance of common stock                                                                                                                                                      | 21            | 22            |
| Mortgage loans                                                                                                                                                                                         | 358           | 234           |
| **Financial Liabilities [Abstract]**                                                                                                                                                                   |          |          |
| Premium deposits and annuity contracts                                                                                                                                                                 | 103           | 109           |
| Short term debt                                                                                                                                                                                        | 13            | 83            |
| Long term debt                                                                                                                                                                                         | 2,557         | 2,525         |
| Estimate Fair Value [Member]                                                                                                                                                                           |          |          |
| **Financial Assets [Abstract]**                                                                                                                                                                        |          |          |
| Notes receivable for the issuance of common stock                                                                                                                                                      | 21            | 22            |
| Mortgage loans                                                                                                                                                                                         | 374           | 247           |
| **Financial Liabilities [Abstract]**                                                                                                                                                                   |          |          |
| Premium deposits and annuity contracts                                                                                                                                                                 | 108           | 114           |
| Short term debt                                                                                                                                                                                        | 13            | 84            |
| Long term debt                                                                                                                                                                                         | 2,986         | 2,679         |
| Estimate Fair Value [Member] | Level 1 [Member]                                                                                                                                                        |          |          |
| **Financial Assets [Abstract]**                                                                                                                                                                        |          |          |
| Notes receivable for the issuance of common stock                                                                                                                                                      |     |          |
| Mortgage loans                                                                                                                                                                                         |     |          |
| **Financial Liabilities [Abstract]**                                                                                                                                                                   |          |          |
| Premium deposits and annuity contracts                                                                                                                                                                 |     |          |
| Short term debt                                                                                                                                                                                        |     |          |
| Long term debt                                                                                                                                                                                         |     |          |
| Estimate Fair Value [Member] | Level 2 [Member]                                                                                                                                                        |          |          |
| **Financial Assets [Abstract]**                                                                                                                                                                        |          |          |
| Notes receivable for the issuance of common stock                                                                                                                                                      |     |          |
| Mortgage loans                                                                                                                                                                                         |     |          |
| **Financial Liabilities [Abstract]**                                                                                                                                                                   |          |          |
| Premium deposits and annuity contracts                                                                                                                                                                 |     |          |
| Short term debt                                                                                                                                                                                        | 13            |          |
| Long term debt                                                                                                                                                                                         | 2,986         |          |
| Estimate Fair Value [Member] | Level 3 [Member]                                                                                                                                                        |          |          |
| **Financial Assets [Abstract]**                                                                                                                                                                        |          |          |
| Notes receivable for the issuance of common stock                                                                                                                                                      | 21            |          |
| Mortgage loans                                                                                                                                                                                         | 374           |          |
| **Financial Liabilities [Abstract]**                                                                                                                                                                   |          |          |
| Premium deposits and annuity contracts                                                                                                                                                                 | 108           |          |
| Short term debt                                                                                                                                                                                        |     |          |
| Long term debt                                                                                                                                                                                         |     |          |

| X                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Premium deposits and annuity contracts
+ References
No definition available.
+ Details**Name:**cna_PremiumDepositsAndAnnuityContracts**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByBalanceSheetGroupingMethodologyFinancialLiabilitiesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total of the portions of the carrying amounts as of the balance sheet date of long-term debt, which may include notes payable, bonds payable, debentures, mortgage loans, and commercial paper, which are scheduled to be repaid within one year or the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20 -Article 5Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Article 5Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19,20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_LongTermDebtCurrent**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Sum of the carrying values as of the balance sheet date of all long-term debt, which is debt initially having maturities due after one year from the balance sheet date or beyond the operating cycle, if longer, but excluding the portions thereof scheduled to be repaid within one year (current maturities) or the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_LongTermDebtNoncurrent**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 60 -Paragraph 47 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 1 -Subparagraph cReference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910
+ Details**Name:**us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amounts receivable from officers and directors resulting from the sale of stock to officers or directors before the cash payment is received.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29,30) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 4 -Section EReference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB TOPIC 4.E) -URI http://asc.fasb.org/extlink&oid=6228006&loc=d3e74512-122707
+ Details**Name:**us-gaap_ReceivableFromOfficersAndDirectorsForIssuanceOfCapitalStock**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |