| **Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions** | 3 Months Ended | 12 Months Ended |
|----------------------------------------------------------------------------------------|----------------|-----------------|
||
| Revenues                                                                               | $ 2,390        | $ 2,433         | $ 2,348 | $ 2,195 | $ 2,269 | $ 2,153 | $ 2,327 | $ 2,352 | $ 9,366 | $ 9,101 | $ 9,692 |
| Specialty [Domain]                                                                     |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 3,662 | 3,579 | 3,708 |
| Management & Professional Liability [Member]                                       |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 2,620 | 2,617 | 2,818 |
| Surety [Member]                                                                        |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 532 | 502 | 509 |
| Warranty & Alternative Risks [Member]                                              |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 510 | 460 | 381 |
| Commercial [Domain]                                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 3,478 | 3,371 | 3,683 |
| Middle Markets [Member]                                                                |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 1,769 | 1,623 | 1,631 |
| Small Business [Member]                                                                |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 583 | 616 | 709 |
| Other Commercial Insurance [Member]                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 1,126 | 1,132 | 1,343 |
| International [Domain]                                                                 |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 875 | 856 | 973 |
| Canada [Member]                                                                        |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 203 | 214 | 273 |
| CNA Europe [Member]                                                                    |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 319 | 309 | 335 |
| Hardy [Member]                                                                         |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 353 | 333 | 365 |
| Life and Group Non-Core [Member]                                                       |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 1,326 | 1,258 | 1,279 |
| Corporate and Other [Member]                                                           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 26 | 42 | 56 |
| Eliminations [Member]                                                                  |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | $ (1) | $ (5) | $ (7) |

| 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=63488584&loc=d3e20235-122688
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| X                                                                                                                                                 |
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| X                                                                                                                                                                       |
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|---------------------------------------------------------------------------------------------------------------------------------------------------|
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|-----------------------------------------------------------------------------------------------------------------------------------------------|
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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