| **Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions** | 3 Months Ended | 12 Months Ended |
|----------------------------------------------------------------------------------------|----------------|-----------------|
||
| Revenues                                                                               | $ 2,448        | $ 2,398         | $ 2,366 | $ 2,330 | $ 2,390 | $ 2,433 | $ 2,348 | $ 2,195 | $ 9,542 | $ 9,366 | $ 9,101 |
| Total operating revenues                                                               |           |            |   |   |   |   |   |   | 9,449 | 9,316 | 9,168 |
| Net realized investment gains (losses)                                                 |           |            |   |   |   |   |   |   | 93 | 50 | (67) |
| Loss on early redemption                                                               | 42             |            |   |   |   |   |   |   | 42 |   |   |
| Management & Professional Liability                                                |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 2,591 | 2,617 | 2,647 |
| Surety                                                                                 |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 541 | 529 | 504 |
| Warranty & Alternative Risks                                                       |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 551 | 511 | 461 |
| Specialty                                                                              |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 3,683 | 3,657 | 3,612 |
| Middle Market                                                                          |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 1,907 | 1,768 | 1,641 |
| Small Business                                                                         |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 480 | 582 | 621 |
| Other Commercial Insurance                                                             |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 1,126 | 1,124 | 1,156 |
| Commercial                                                                             |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 3,513 | 3,474 | 3,418 |
| Canada                                                                                 |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 224 | 203 | 213 |
| CNA Europe                                                                             |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 326 | 314 | 311 |
| Hardy                                                                                  |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 359 | 340 | 331 |
| International                                                                          |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 909 | 857 | 855 |
| Life & Group                                                                       |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 1,323 | 1,301 | 1,259 |
| Corporate & Other                                                                  |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | 22 | 28 | 29 |
| Eliminations                                                                           |           |            |   |   |   |   |   |   |   |   |   |
| Revenues                                                                               |           |            |   |   |   |   |   |   | (1) | $ (1) | $ (5) |
| Senior Notes, Due November 2019                                                        |           |            |   |   |   |   |   |   |   |   |   |
| Face amount                                                                            | $ 350          |            |   |   |   |   |   |   | $ 350 |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Face (par) amount of debt instrument at time of issuance.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of early contract termination fees included in liabilities subject to compromise.
+ References
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| 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
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.
+ References
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+ Details**Name:**us-gaap_SalesRevenueNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
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