| **Investments (Components of other-than-temporary impairment losses recognized in earnings) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 6 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Net OTTI Losses**                                                                                                                                    |           |           |   |   |
| Net other-than-temporary impairment losses recognized in earnings                                                                                      | $ 23           | $ 62           | $ 50 | $ 103 |
| Corporate and other bonds [Member]                                                                                                                     |           |           |   |   |
| **Net OTTI Losses**                                                                                                                                    |           |           |   |   |
| Net other-than-temporary impairment losses recognized in earnings                                                                                      | 6              | 15             | 16 | 24 |
| Residential mortgage-backed [Member]                                                                                                                   |           |           |   |   |
| **Net OTTI Losses**                                                                                                                                    |           |           |   |   |
| Net other-than-temporary impairment losses recognized in earnings                                                                                      | 15             | 46             | 29 | 74 |
| U.S. Treasury and obligations of government-sponsored enterprises [Member]                                                                             |           |           |   |   |
| **Net OTTI Losses**                                                                                                                                    |           |           |   |   |
| Net other-than-temporary impairment losses recognized in earnings                                                                                      |      |      | 1 |    |
| Total fixed maturity securities available-for-sale [Member]                                                                                            |           |           |   |   |
| **Net OTTI Losses**                                                                                                                                    |           |           |   |   |
| Net other-than-temporary impairment losses recognized in earnings                                                                                      | 21             | 61             | 46 | 98 |
| Common stock [Member]                                                                                                                                  |           |           |   |   |
| **Net OTTI Losses**                                                                                                                                    |           |           |   |   |
| Net other-than-temporary impairment losses recognized in earnings                                                                                      | 2              | 1              | 4 | 4 |
| Preferred stock [Member]                                                                                                                               |           |           |   |   |
| **Net OTTI Losses**                                                                                                                                    |           |           |   |   |
| Net other-than-temporary impairment losses recognized in earnings                                                                                      |      |      |    | 1 |
| Total equity securities available-for-sale [Member]                                                                                                    |           |           |   |   |
| **Net OTTI Losses**                                                                                                                                    |           |           |   |   |
| Net other-than-temporary impairment losses recognized in earnings                                                                                      | $ 2            | $ 1            | $ 4 | $ 5 |

| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cna_NetOttiLossesAbstract**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
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| - Definition
The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS115-1/124-1 -Paragraph 16B -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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS115-1/124-1 -Paragraph 7 -Footnote 2 -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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS115-1/124-1 -Paragraph 15C -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 FASB Staff Position (FSP) -Number FAS115-1/124-1 -Paragraph 15D -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.
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