| **Business Segments (Revenues by Line of Business) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| Revenues                                                                                                      | $ 2,434        | $ 2,466         | $ 2,246 | $ 2,401 | $ 2,261 | $ 2,175 | $ 2,198 | $ 2,315 | $ 9,547 | $ 8,949 | $ 9,209 |
| CNA Specialty [Domain]                                                                                        |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 3,742 | 3,512 | 3,516 |
| International - Specialty [Member]                                                                            |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 220 | 210 | 199 |
| Professional & Management Liability [Member]                                                              |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 2,723 | 2,541 | 2,551 |
| Surety [Member]                                                                                               |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 485 | 472 | 475 |
| Warranty & Alternative Risks [Member]                                                                     |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 314 | 289 | 291 |
| CNA Commercial [Domain]                                                                                       |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 4,238 | 4,073 | 4,175 |
| CNA Select Risk [Member]                                                                                      |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 272 | 272 | 261 |
| Commercial Insurance [Member]                                                                                 |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 2,928 | 2,681 | 2,851 |
| International - Commercial [Member]                                                                           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 369 | 539 | 499 |
| Small Business [Member]                                                                                       |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 669 | 581 | 564 |
| Hardy [Domain]                                                                                                |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 123 |   |   |
| Life & Group Non-Core [Domain]                                                                            |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 1,395 | 1,334 | 1,357 |
| Health [Member]                                                                                               |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 1,120 | 1,093 | 1,100 |
| Life & Annuity [Member]                                                                                   |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 239 | 229 | 249 |
| Other                                                                                                         |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 36 | 12 | 8 |
| Corporate and Other Non-Core revenues [Member]                                                                |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 52 | 33 | 164 |
| Eliminations [Member]                                                                                         |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | $ (3) | $ (3) | $ (3) |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate revenue recognized during the period (derived from goods sold, services rendered, insurance premiums, or other activities that constitute an entity's earning process). For financial services companies, also includes investment and interest income, and sales and trading gains.
+ 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=6880815&loc=d3e20235-122688Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Article 5
+ Details**Name:**us-gaap_Revenues**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |