| **Legal Proceedings and Contingent Liabilities**            | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Legal Proceedings and Contingent Liabilities [Abstract]** | Â                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
| Legal Matters and Contingencies [Text Block]                | Legal Proceedings and Contingent LiabilitiesInsurance Brokerage Antitrust LitigationIn August 2005, CNAF and certain insurance subsidiaries were joined as defendants, along with other insurers and brokers, in multidistrict litigation pending in the United States District Court for the District of New Jersey, In re Insurance Brokerage Antitrust Litigation, Civil No. 04-5184 (GEB). The plaintiffs' consolidated class action complaint alleges bid rigging and improprieties in the payment of contingent commissions in connection with the sale of insurance that violated federal and state antitrust laws, the federal Racketeer Influenced and Corrupt Organizations (RICO) Act and state common law. After discovery, the District Court dismissed the federal antitrust claims and the RICO claims, and declined to exercise supplemental jurisdiction over the state law claims. The plaintiffs appealed the dismissal of their complaint to the Third Circuit Court of Appeals. In August 2010, the Court of Appeals affirmed the District Court's dismissal of the antitrust claims and the RICO claims against CNAF and certain insurance subsidiaries, but vacated the dismissal of one portion of those claims against some other parties and remanded them for further proceedings on motions to dismiss.  The Court of Appeals also vacated and remanded the dismissal of the state law claims against CNAF and certain insurance subsidiaries and other parties to allow for further proceedings relating to motions to dismiss before the District Court.  In November 2010, CNAF and certain insurance subsidiaries filed in the district court a motion to dismiss the remaining state law claims pending against them. In March 2011, CNAF and certain insurance subsidiaries, along with certain other defendants, entered into a memorandum of settlement understanding with the plaintiffs to settle all claims asserted, or which could have been asserted, in the class action lawsuit.  After negotiating additional terms, the parties executed final settlement documents and the plaintiffs filed a motion for preliminary approval of the settlement in May 2011. In June 2011, the Court entered an order preliminarily approving the settlement. A fairness hearing is scheduled in September 2011 to determine final settlement, after providing notice to the class and an opportunity for any objections to be heard. As currently structured, the settlement will not have a material impact on the Company's results of operations. In addition, the Company does not believe it has any material ongoing exposure relating to this matter.Other LitigationThe Company is also a party to routine litigation incidental to its business, which, based on the facts and circumstances currently known, is not material to the business or financial condition of the Company. |

| X                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Legal Proceedings and Contingent Liabilities [Abstract]
+ Details**Name:**cna_LegalProceedingsAndContingentLiabilitiesAbstract**Namespace Prefix:**cna**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The entire disclosure for legal proceedings, legal contingencies, litigation, regulatory and environmental matters and other contingencies.
+ Details**Name:**us-gaap_LegalMattersAndContingenciesTextBlock**Namespace Prefix:**us-gaap**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |