| **Quarterly Financial Data (Unaudited)** | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
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| **Quarterly Financial Data [Abstract]**  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
| Quarterly Financial Data (Unaudited)     | Note Q. Quarterly Financial Data (Unaudited)The following tables set forth unaudited quarterly financial data for the years ended December 31, 2012 and 2011.Quarterly Financial Data2012         (In millions, except per share data)First Second Third Fourth Full YearRevenues$2,401 $2,246 $2,466 $2,434 $9,547Net income 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) $2.332011         (In millions, except per share data)First Second Third Fourth Full YearRevenues$2,315 $2,198 $2,175 $2,261 $8,949Income (loss) from continuing operations230 129 76 194 629Income (loss) from discontinued operations, net of income tax (expense) benefit(1) — — — (1)Net (income) loss attributable to noncontrolling interests(9) (5) (1) (1) (16)Net income (loss) attributable to CNA$220 $124 $75 $193 $612Basic and Diluted Earnings (Loss) Per Share Attributable to CNA Common Stockholders         Income (loss) from continuing operations attributable to CNA common stockholders$0.82 $0.46 $0.28 $0.71 $2.27Income (loss) from discontinued operations attributable to CNA common stockholders— — — — —Basic and diluted earnings (loss) per share attributable to CNA common stockholders$0.82 $0.46 $0.28 $0.71 $2.27____________________(a)Due to the averaging of shares, quarterly earnings per share do not add to the total for the full year.During the fourth quarter of 2012, catastrophe impacts incurred, net of reinsurance and including reinstatement premiums, were $280 million related to Storm Sandy. |

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| - Definition
The 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.
+ References
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