| **Schedule IV. Reinsurance**            | 12 Months Ended                                                                                                                               |
|-----------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Schedule IV. Reinsurance [Abstract]** |                                                                                                                                          |
| Schedule IV. Reinsurance                | SCHEDULE IV.  REINSURANCEIncorporated herein by reference to Note H to the Consolidated Financial Statements included under Item 8. |

| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cna_ScheduleIvReinsuranceAbstract**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The entire disclosure for the  supplemental schedule of reinsurance information required to be filed for each period for which an audited income statement is presented. Reinsurance schedule includes the type of insurance, gross amount of premiums, premiums ceded to other companies, premiums assumed from other companies, net amount of premium revenue, and the percentage of the premiums assumed to net premium revenue.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.7-05.(c) Schedule IV) -URI http://asc.fasb.org/extlink&oid=6879921&loc=d3e569147-122900Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 05 -Article 7Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17) -URI http://asc.fasb.org/extlink&oid=6588446&loc=d3e574992-122915
+ Details**Name:**us-gaap_SupplementalScheduleOfReinsurancePremiumsForInsuranceCompaniesTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |