| **Accumulated Other Comprehensive Income (Loss) (Tables)**   | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Accumulated Other Comprehensive Income (Loss) [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| Schedule of accumulated other comprehensive income (loss)    | Accumulated Other Comprehensive Income (Loss)December 312012 2011(In millions)Tax After-tax Tax After-taxCumulative foreign currency translation adjustment$— $161 $— $121Pension and postretirement benefits387 (721) 326 (609)Net unrealized gains (losses) on investments with OTTI losses(11) 20 33 (64)Net unrealized gains (losses) on other investments(721) 1,371 (538) 1,032Accumulated other comprehensive income (loss)$(345) $831 $(179) $480 |

| X                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cna_AccumulatedOtherComprehensiveIncomeLossAbstract**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of the components of accumulated other comprehensive income (loss).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=20435746&loc=SL7669686-108580
+ Details**Name:**us-gaap_ScheduleOfAccumulatedOtherComprehensiveIncomeLossTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |