| **Quarterly Financial Data (Unaudited) (Tables)** | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
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| **Quarterly Financial Data [Abstract]**           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
| Schedule of quarterly financial information       | Quarterly Financial Data2013         (In millions, except per share data)First Second Third Fourth Full 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)First Second Third Fourth Full 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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| - Definition
Tabular disclosure of the quarterly financial data in the annual financial statements. The disclosure includes financial information for each fiscal quarter for the current and previous year, including revenues, gross profit, income (loss) before extraordinary items and cumulative effect of a change in accounting principle and earnings per share data.
+ References
Reference 1: 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=d3e1280-108306
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