| **Business Segments - Revenues by Line of Business (Details) - USD ($) $ in Millions** | 3 Months Ended |
|----------------------------------------------------------------------------------------|----------------|
||
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | $ 2,330        | $ 2,195 |
| Operating Segments | Specialty                                                         |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 918            | 865 |
| Operating Segments | Commercial                                                        |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 849            | 802 |
| Operating Segments | International                                                     |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 215            | 215 |
| Operating Segments | Life & Group Non-Core                                         |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 341            | 315 |
| Operating Segments | Corporate & Other Non-Core                                    |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 7              | (2) |
| Intersegment Eliminations                                                              |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 0              | 0 |
| Specialty | Operating Segments                                                         |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 661            | 618 |
| Surety | Operating Segments                                                            |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 123            | 127 |
| Warranty & Alternative Risks | Operating Segments                                  |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 134            | 120 |
| Middle Market | Operating Segments                                                     |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 458            | 401 |
| Small Business | Operating Segments                                                    |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 97             | 143 |
| Other Commercial Insurance | Operating Segments                                        |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 294            | 258 |
| Canada | Operating Segments                                                            |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 51             | 50 |
| CNA Europe | Operating Segments                                                        |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | 73             | 78 |
| Hardy | Operating Segments                                                             |           |   |
| **Segment Reporting Information [Line Items]**                                         |           |   |
| Revenues                                                                               | $ 91           | $ 87 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).
+ 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=63488584&loc=d3e20235-122688
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| X                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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