|**Quarterly Financial Data (Unaudited)**                                                | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|**Quarterly Financial Data [Abstract]**                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Quarterly Financial Data (Unaudited)                                                    | NoteP. Quarterly Financial Data (Unaudited)The following tables set forth unaudited quarterly financial data for the years endedDecember 31, 2013and2012.Quarterly Financial Data2013(In millions, except per share data)FirstSecondThirdFourthFull YearRevenues$2,503$2,493$2,504$2,613$10,113Net income (loss) attributable to CNA$250$194$272$221$937Basic earnings (loss) per share attributable to CNA common stockholders$0.93$0.72$1.01$0.82$3.48Diluted earnings (loss) per share attributable to CNA common stockholders (a)$0.93$0.72$1.01$0.82$3.472012(In millions, except per share data)FirstSecondThirdFourthFull YearRevenues$2,401$2,246$2,466$2,434$9,547Net income (loss) attributable to CNA$250$166$221$(9)$628Basic and diluted earnings (loss) per share attributable to CNA common stockholders (a)$0.93$0.62$0.82$(0.03)$2.33____________________(a)Due to the averaging of shares, quarterly earnings per share do not add to the total for the full year.
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|2013                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|(In millions, except per share data)                                                    | First                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Revenues                                                                                | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Net income (loss) attributable to CNA                                                   | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Basic earnings (loss) per share attributable to CNA common stockholders                 | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Diluted earnings (loss) per share attributable to CNA common stockholders (a)           | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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|2012                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|(In millions, except per share data)                                                    | First                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Revenues                                                                                | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Net income (loss) attributable to CNA                                                   | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Basic and diluted earnings (loss) per share attributable to CNA common stockholders (a) | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
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|(a)                                                                                     | Due to the averaging of shares, quarterly earnings per share do not add to the total for the full year.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             

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                    duration X- DefinitionThe entire disclosure for the quarterly financial data in the annual financial statements.  The disclosure may include a tabular presentation of financial information for each fiscal quarter for the current and previous year, including revenues, gross profit, income or loss before extraordinary items and earnings per share data. It also includes an indication if the information in the note is unaudited, comments on the aggregate effect of year-end adjustments, and an explanation of matters or transactions that affect comparability or are pertinent to an understanding of the information furnished.+ ReferencesReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 270 -SubTopic 10 -Section 45 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6372559&loc=d3e765-108305Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 270 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6372559&loc=d3e725-108305Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 270 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a)-(j) -URI http://asc.fasb.org/extlink&oid=25249566&loc=d3e1280-108306Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section G -Subsection 1Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-K (SK) -Number 229 -Section 302 -Paragraph a+ Details**Name:**
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