| **Business Segments (Revenues by Line of Business) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended |
|---------------------------------------------------------------------------------------------------------------|----------------|
||
| Revenues                                                                                                      | $ 2,503        | $ 2,401 |
| CNA Specialty [Domain]                                                                                        |           |   |
| Revenues                                                                                                      | 956            | 945 |
| International - Specialty [Member]                                                                            |           |   |
| Revenues                                                                                                      | 59             | 57 |
| Professional & Management Liability [Member]                                                              |           |   |
| Revenues                                                                                                      | 700            | 694 |
| Surety [Member]                                                                                               |           |   |
| Revenues                                                                                                      | 116            | 119 |
| Warranty & Alternative Risks [Member]                                                                     |           |   |
| Revenues                                                                                                      | 81             | 75 |
| CNA Commercial [Domain]                                                                                       |           |   |
| Revenues                                                                                                      | 1,101          | 1,088 |
| Commercial Insurance [Member]                                                                                 |           |   |
| Revenues                                                                                                      | 826            | 835 |
| International - Commercial [Member]                                                                           |           |   |
| Revenues                                                                                                      | 93             | 91 |
| Small Business [Member]                                                                                       |           |   |
| Revenues                                                                                                      | 182            | 162 |
| Hardy [Domain]                                                                                                |           |   |
| Revenues                                                                                                      | 67             |   |
| Life & Group Non-Core [Domain]                                                                            |           |   |
| Revenues                                                                                                      | 363            | 350 |
| Health [Member]                                                                                               |           |   |
| Revenues                                                                                                      | 298            | 291 |
| Life & Annuity [Member]                                                                                   |           |   |
| Revenues                                                                                                      | 61             | 60 |
| Other                                                                                                         |           |   |
| Revenues                                                                                                      | 4              | (1) |
| Corporate and Other Non-Core revenues [Member]                                                                |           |   |
| Revenues                                                                                                      | 17             | 18 |
| Eliminations [Member]                                                                                         |           |   |
| Revenues                                                                                                      | $ (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 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 |