|**Stockholders' Equity and Statutory Accounting Practices (Tables)**   | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                         
|---------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Stockholders' Equity and Statutory Accounting Practices [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                         
|Combined statutory capital and surplus and net income (loss)           | Statutory InformationStatutory Capital and SurplusStatutory Net Income (Loss)December 31Years ended December 31(In millions)2011 (b)20102011 (b)20102009Combined Continental Casualty Companies (a)$9,888$9,821$954$258$17Life company5194982986(65)________________(a)Represents the combined statutory surplus of CCC and its subsidiaries, including the Life company.(b)Preliminary.
|                                                                      
|                                                                       |                                                                                                                                                                                                                                                                                                                                                                                         
|                                                                       | Statutory Capital and Surplus                                                                                                                                                                                                                                                                                                                                                           
|                                                                       | December 31                                                                                                                                                                                                                                                                                                                                                                             
|(In millions)                                                          | 2011 (b)                                                                                                                                                                                                                                                                                                                                                                                
|Combined Continental Casualty Companies (a)                            | $                                                                                                                                                                                                                                                                                                                                                                                       
|Life company                                                           | 519                                                                                                                                                                                                                                                                                                                                                                                     
|                                                                       |                                                                                                                                                                                                                                                                                                                                                                                         
|(a)                                                                    | Represents the combined statutory surplus of CCC and its subsidiaries, including the Life company.                                                                                                                                                                                                                                                                                      
|                                                                       |                                                                                                                                                                                                                                                                                                                                                                                         
|(b)                                                                    | Preliminary.                                                                                                                                                                                                                                                                                                                                                                            

X- DefinitionSchedule of statutory net income and capital and surplus, determined in accordance with statutory accounting practices prescribed or permitted by insurance regulatory authorities.+ ReferencesNo definition available.+ Details**Name:**
                    cna_ScheduleOfStatutoryFinancialInformationTableTextBlock**Namespace Prefix:**
                    cna_**Data Type:**
                    nonnum:textBlockItemType**Balance Type:**
                    na**Period Type:**
                    duration X- Details**Name:**
                    cna_StockholdersEquityAndStatutoryAccountingPracticesAbstract**Namespace Prefix:**
                    cna_**Data Type:**
                    xbrli:stringItemType**Balance Type:**
                    na**Period Type:**
                    duration