| **Business Segments (Revenues by Line of Business) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| Revenues                                                                                                      | $ 2,466        | $ 2,175        | $ 7,113 | $ 6,688 |
| CNA Specialty [Domain]                                                                                        |           |           |   |   |
| Revenues                                                                                                      | 957            | 874            | 2,798 | 2,640 |
| International - Specialty [Member]                                                                            |           |           |   |   |
| Revenues                                                                                                      | 53             | 54             | 163 | 159 |
| Professional & Management Liability [Member]                                                              |           |           |   |   |
| Revenues                                                                                                      | 700            | 623            | 2,039 | 1,909 |
| Surety [Member]                                                                                               |           |           |   |   |
| Revenues                                                                                                      | 124            | 123            | 364 | 353 |
| Warranty & Alternative Risks [Member]                                                                     |           |           |   |   |
| Revenues                                                                                                      | 80             | 74             | 232 | 219 |
| CNA Commercial [Domain]                                                                                       |           |           |   |   |
| Revenues                                                                                                      | 1,091          | 962            | 3,163 | 3,040 |
| CNA Select Risk [Member]                                                                                      |           |           |   |   |
| Revenues                                                                                                      | 70             | 65             | 208 | 203 |
| Commercial Insurance [Member]                                                                                 |           |           |   |   |
| Revenues                                                                                                      | 756            | 622            | 2,188 | 2,015 |
| International - Commercial [Member]                                                                           |           |           |   |   |
| Revenues                                                                                                      | 93             | 133            | 274 | 390 |
| Small Business [Member]                                                                                       |           |           |   |   |
| Revenues                                                                                                      | 172            | 142            | 493 | 432 |
| Hardy [Domain]                                                                                                |           |           |   |   |
| Revenues                                                                                                      | 65             |      | 65 |    |
| Life & Group Non-Core [Domain]                                                                            |           |           |   |   |
| Revenues                                                                                                      | 340            | 334            | 1,050 | 993 |
| Health [Member]                                                                                               |           |           |   |   |
| Revenues                                                                                                      | 274            | 274            | 852 | 818 |
| Life & Annuity [Member]                                                                                   |           |           |   |   |
| Revenues                                                                                                      | 64             | 55             | 181 | 170 |
| Other - Life & Group [Member]                                                                             |           |           |   |   |
| Revenues                                                                                                      | 2              | 5              | 17 | 5 |
| Corporate and Other Non-Core [Member]                                                                         |           |           |   |   |
| Revenues                                                                                                      | 14             | 6              | 39 | 17 |
| Eliminations [Member]                                                                                         |           |           |   |   |
| Revenues                                                                                                      | $ (1)          | $ (1)          | $ (2) | $ (2) |

| 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 |