| **Business Segments (Revenues by Line of Business) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| Revenues                                                                                                      | $ 2,259        | $ 2,175         | $ 2,198 | $ 2,315 | $ 2,298 | $ 2,363 | $ 2,233 | $ 2,315 | $ 8,947 | $ 9,209 | $ 8,472 |
| CNA Specialty [Domain]                                                                                        |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 3,512 | 3,516 | 3,243 |
| International - Specialty [Member]                                                                            |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 210 | 199 | 171 |
| Professional & Management Liability [Member]                                                              |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 2,541 | 2,551 | 2,339 |
| Surety [Member]                                                                                               |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 472 | 475 | 474 |
| Warranty & Alternative Risks [Member]                                                                     |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 289 | 291 | 259 |
| CNA Commercial [Domain]                                                                                       |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 4,071 | 4,175 | 4,068 |
| CNA Select Risk [Member]                                                                                      |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 272 | 261 | 210 |
| Commercial Insurance [Member]                                                                                 |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 2,681 | 2,851 | 2,692 |
| International - Commercial [Member]                                                                           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 537 | 499 | 617 |
| Small Business [Member]                                                                                       |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 581 | 564 | 549 |
| Life & Group Non-Core [Domain]                                                                            |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 1,334 | 1,357 | 1,035 |
| Health [Member]                                                                                               |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 1,093 | 1,100 | 811 |
| Life & Annuity [Member]                                                                                   |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 229 | 249 | 211 |
| Other - Life & Group [Member]                                                                             |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 12 | 8 | 13 |
| Corporate and Other Non-Core [Member]                                                                         |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                                                      |           |            |   |   |   |   |   |   | 33 | 164 | 129 |
| 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 SEC -Name Regulation S-X (SX) -Number 210 -Article 5 -Section 03 -Paragraph 1Reference 2: 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=d3e20235-122688
+ Details**Name:**us-gaap_Revenues**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |