| **Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions** | 3 Months Ended | 12 Months Ended |
|----------------------------------------------------------------------------------------|----------------|-----------------|
||
| Revenues                                                                               | $ 2,269        | $ 2,153         | $ 2,327 | $ 2,352 | $ 2,378 | $ 2,411 | $ 2,440 | $ 2,463 | $ 9,101 | $ 9,692 | $ 9,932 |
| Specialty [Domain]                                                                     |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 3,579 | 3,708 | 3,676 |
| Management & Professional Liability [Member]                                       |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 2,617 | 2,818 | 2,836 |
| Surety [Member]                                                                        |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 502 | 509 | 490 |
| Warranty & Alternative Risks [Member]                                              |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 460 | 381 | 350 |
| Commercial [Domain]                                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 3,371 | 3,683 | 3,984 |
| Middle Markets [Member]                                                                |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 1,623 | 1,631 | 1,642 |
| Small Business [Member]                                                                |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 616 | 709 | 754 |
| Other Commercial Insurance [Member]                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 1,132 | 1,343 | 1,588 |
| International [Domain]                                                                 |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 856 | 973 | 981 |
| Canada [Member]                                                                        |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 214 | 273 | 289 |
| CNA Europe [Member]                                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 309 | 335 | 326 |
| Hardy [Member]                                                                         |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 333 | 365 | 366 |
| Life and Group Non-Core [Member]                                                       |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 1,258 | 1,279 | 1,243 |
| Corporate and Other Non-Core [Member]                                                  |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 42 | 56 | 52 |
| Eliminations [Member]                                                                  |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | $ (5) | $ (7) | $ (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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|-----------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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