| **Business Segments (Revenues by Line of Business) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 6 Months Ended |
|---------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| Revenues                                                                                                      | $ 2,246        | $ 2,198        | $ 4,647 | $ 4,513 |
| CNA Specialty [Domain]                                                                                        |           |           |   |   |
| Revenues                                                                                                      | 896            | 875            | 1,841 | 1,766 |
| International - Specialty [Member]                                                                            |           |           |   |   |
| Revenues                                                                                                      | 53             | 54             | 110 | 105 |
| Professional & Management Liability [Member]                                                              |           |           |   |   |
| Revenues                                                                                                      | 645            | 626            | 1,339 | 1,286 |
| Surety [Member]                                                                                               |           |           |   |   |
| Revenues                                                                                                      | 121            | 119            | 240 | 230 |
| Warranty & Alternative Risks [Member]                                                                     |           |           |   |   |
| Revenues                                                                                                      | 77             | 76             | 152 | 145 |
| CNA Commercial [Domain]                                                                                       |           |           |   |   |
| Revenues                                                                                                      | 984            | 984            | 2,072 | 2,078 |
| CNA Select Risk [Member]                                                                                      |           |           |   |   |
| Revenues                                                                                                      | 66             | 67             | 139 | 138 |
| Commercial Insurance [Member]                                                                                 |           |           |   |   |
| Revenues                                                                                                      | 670            | 641            | 1,432 | 1,393 |
| International - Commercial [Member]                                                                           |           |           |   |   |
| Revenues                                                                                                      | 90             | 130            | 181 | 257 |
| Small Business [Member]                                                                                       |           |           |   |   |
| Revenues                                                                                                      | 158            | 146            | 320 | 290 |
| Life & Group Non-Core [Domain]                                                                            |           |           |   |   |
| Revenues                                                                                                      | 360            | 333            | 710 | 659 |
| Health [Member]                                                                                               |           |           |   |   |
| Revenues                                                                                                      | 287            | 274            | 578 | 544 |
| Life & Annuity [Member]                                                                                   |           |           |   |   |
| Revenues                                                                                                      | 57             | 59             | 117 | 115 |
| Other - Life & Group [Member]                                                                             |           |           |   |   |
| Revenues                                                                                                      | 16             |      | 15 |    |
| Corporate and Other Non-Core [Member]                                                                         |           |           |   |   |
| Revenues                                                                                                      | 7              | 6              | 25 | 11 |
| Eliminations [Member]                                                                                         |           |           |   |   |
| Revenues                                                                                                      | $ (1)          |      | $ (1) | $ (1) |

| 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 |