|**Schedule V. Valuation and Qualifying Accounts (Tables)** | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|---------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Valuation and Qualifying Accounts [Abstract]**           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Schedule of valuation and qualifying accounts              | SCHEDULE V. VALUATION AND QUALIFYING ACCOUNTS(In millions)Balance at Beginning of PeriodCharged to Costs and ExpensesCharged to Other Accounts (a)DeductionsBalance at End of PeriodYear ended December 31, 2012Deducted from assets:Allowance for doubtful accounts:Insurance and reinsurance receivables$203$(23)$5$(11)$174Year ended December 31, 2011Deducted from assets:Allowance for doubtful accounts:Insurance and reinsurance receivables$285$(55)$—$(27)$203Year ended December 31, 2010Deducted from assets:Allowance for doubtful accounts:Insurance and reinsurance receivables$553$(232)$(1)$(35)$285(a)    Amount includes effects of foreign currency translation.
|                                                          
|                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|(In millions)                                              | Balance at Beginning of Period                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Year ended December 31, 2012                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Deducted from assets:                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Allowance for doubtful accounts:                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Insurance and reinsurance receivables                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Year ended December 31, 2011                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Deducted from assets:                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Allowance for doubtful accounts:                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Insurance and reinsurance receivables                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Year ended December 31, 2010                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Deducted from assets:                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Allowance for doubtful accounts:                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Insurance and reinsurance receivables                      | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   

X- DefinitionThe tabular disclosure for any allowance and reserve accounts (their beginning and ending balances, as well as a reconciliation by type of activity during the period). Alternatively, disclosure of the required information may be within the footnotes to the financial statements or a supplemental schedule to the financial statements.+ ReferencesNo definition available.+ Details**Name:**
                    cna_ScheduleOfValuationAndQualifyingAccountsTableTextBlock**Namespace Prefix:**
                    cna_**Data Type:**
                    nonnum:textBlockItemType**Balance Type:**
                    na**Period Type:**
                    duration X- Details**Name:**
                    us-gaap_ValuationAndQualifyingAccountsAbstract**Namespace Prefix:**
                    us-gaap_**Data Type:**
                    xbrli:stringItemType**Balance Type:**
                    na**Period Type:**
                    duration