| **Earnings Per Share**                   | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
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| **Earnings (Loss) Per Share [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
| Earnings Per Share [Text Block]          | Note C. Earnings Per ShareEarnings per share attributable to the Company's common stockholders is based on the weighted average number of outstanding common shares.  Basic earnings (loss) per share excludes the impact of dilutive securities and is computed by dividing net income (loss) attributable to CNA by the weighted average number of common shares outstanding for the period.  Diluted earnings (loss) per share reflects the potential dilution that could occur if securities or other contracts to issue common stock were exercised or converted into common stock.For the three and nine months ended September 30, 2012, approximately 450 thousandand 400 thousand potential shares attributable to exercises under stock-based employee compensation plans were included in the calculation of diluted earnings per share.  For those same periods, approximately 693 thousandand740 thousand potential shares attributable to exercises under stock-based employee compensation plans were not included in the calculation of diluted earnings per share because the effect would have been antidilutive.For the three and nine months ended September 30, 2011, approximately 279 thousand and 286 thousand potential shares attributable to exercises under stock-based employee compensation plans were included in the calculation of diluted earnings per share.  For those same periods, approximately 1.2 million and 1.1 million potential shares attributable to exercises under stock-based employee compensation plans were not included in the calculation of diluted earnings per share because the effect would have been antidilutive. |

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| - Definition
The entire disclosure for earnings per share.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6920599&loc=d3e1252-109256Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 55 -Paragraph 52 -URI http://asc.fasb.org/extlink&oid=6945512&loc=d3e4984-109258Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 128 -Paragraph 40 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6920599&loc=d3e1278-109256Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.21) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688
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