| **Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|----------------------------------------------------------------------------------------|----------------|----------------|
||
| Revenues                                                                               | $ 2,433        | $ 2,153        | $ 6,976 | $ 6,832 |
| Specialty [Domain]                                                                     |           |           |   |   |
| Revenues                                                                               | 946            | 846            | 2,739 | 2,667 |
| Management & Professional Liability [Member]                                       |           |           |   |   |
| Revenues                                                                               | 677            | 605            | 1,954 | 1,976 |
| Surety [Member]                                                                        |           |           |   |   |
| Revenues                                                                               | 139            | 132            | 399 | 377 |
| Warranty & Alternative Risks [Member]                                              |           |           |   |   |
| Revenues                                                                               | 130            | 109            | 386 | 314 |
| Commercial [Domain]                                                                    |           |           |   |   |
| Revenues                                                                               | 913            | 767            | 2,591 | 2,545 |
| Middle Markets [Member]                                                                |           |           |   |   |
| Revenues                                                                               | 463            | 386            | 1,298 | 1,207 |
| Small Business [Member]                                                                |           |           |   |   |
| Revenues                                                                               | 154            | 144            | 448 | 467 |
| Other Commercial Insurance [Member]                                                    |           |           |   |   |
| Revenues                                                                               | 296            | 237            | 845 | 871 |
| International [Domain]                                                                 |           |           |   |   |
| Revenues                                                                               | 229            | 216            | 658 | 642 |
| Canada [Member]                                                                        |           |           |   |   |
| Revenues                                                                               | 51             | 52             | 152 | 161 |
| CNA Europe [Member]                                                                    |           |           |   |   |
| Revenues                                                                               | 82             | 78             | 241 | 232 |
| Hardy [Member]                                                                         |           |           |   |   |
| Revenues                                                                               | 96             | 86             | 265 | 249 |
| Life and Group Non-Core [Member]                                                       |           |           |   |   |
| Revenues                                                                               | 339            | 320            | 979 | 958 |
| Corporate and Other Non-Core [Member]                                                  |           |           |   |   |
| Revenues                                                                               | 6              | 6              | 10 | 24 |
| Eliminations [Member]                                                                  |           |           |   |   |
| Revenues                                                                               | $ 0            | $ (2)          | $ (1) | $ (4) |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688
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| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                       |
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| X                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                             |
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| X                                                                                                                                                                   |
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