| **Business Segments (Revenues by Line of Business) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| Revenues                                                                                                      | $ 2,504        | $ 2,466        | $ 7,500 | $ 7,113 |
| CNA Specialty [Domain]                                                                                        |           |           |   |   |
| Revenues                                                                                                      | 996            | 957            | 2,906 | 2,798 |
| International - Specialty [Member]                                                                            |           |           |   |   |
| Revenues                                                                                                      | 59             | 53             | 177 | 163 |
| Professional & Management Liability [Member]                                                              |           |           |   |   |
| Revenues                                                                                                      | 720            | 700            | 2,107 | 2,039 |
| Surety [Member]                                                                                               |           |           |   |   |
| Revenues                                                                                                      | 126            | 124            | 365 | 364 |
| Warranty & Alternative Risks [Member]                                                                     |           |           |   |   |
| Revenues                                                                                                      | 91             | 80             | 257 | 232 |
| CNA Commercial [Domain]                                                                                       |           |           |   |   |
| Revenues                                                                                                      | 1,062          | 1,091          | 3,267 | 3,163 |
| Commercial Insurance [Member]                                                                                 |           |           |   |   |
| Revenues                                                                                                      | 776            | 826            | 2,429 | 2,396 |
| International - Commercial [Member]                                                                           |           |           |   |   |
| Revenues                                                                                                      | 93             | 93             | 276 | 274 |
| Small Business [Member]                                                                                       |           |           |   |   |
| Revenues                                                                                                      | 193            | 172            | 562 | 493 |
| Hardy [Domain]                                                                                                |           |           |   |   |
| Revenues                                                                                                      | 88             | 65             | 232 | 65 |
| Life & Group Non-Core [Domain]                                                                            |           |           |   |   |
| Revenues                                                                                                      | 346            | 340            | 1,055 | 1,050 |
| Health [Member]                                                                                               |           |           |   |   |
| Revenues                                                                                                      | 290            | 274            | 875 | 852 |
| Life & Annuity [Member]                                                                                   |           |           |   |   |
| Revenues                                                                                                      | 58             | 64             | 177 | 181 |
| Other                                                                                                         |           |           |   |   |
| Revenues                                                                                                      | (2)            | 2              | 3 | 17 |
| Corporate and Other Non-Core revenues [Member]                                                                |           |           |   |   |
| Revenues                                                                                                      | 13             | 14             | 43 | 39 |
| Eliminations [Member]                                                                                         |           |           |   |   |
| Revenues                                                                                                      | $ (1)          | $ (1)          | $ (3) | $ (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 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 |