| **Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $)In Millions, unless otherwise specified**      | 3 Months Ended | 9 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | $ (40)         | $ (36)         | $ (110) | $ (166) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (7)            | (36)           | (55) | (13) |
| Total pretax favorable unfavorable net prior year development, excluding Life & Group                                                  | (47)           | (72)           | (165) | (179) |
| CNA Specialty [Member]                                                                                                                     |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (39)           | (5)            | (80) | (72) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (1)            | (26)           | (15) | (34) |
| Total pretax favorable unfavorable net prior year development, excluding Life & Group                                                  | (40)           | (31)           | (95) | (106) |
| CNA Commercial [Member]                                                                                                                    |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | 2              | (42)           | (25) | (99) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (5)            | (11)           | (41) | 21 |
| Total pretax favorable unfavorable net prior year development, excluding Life & Group                                                  | (3)            | (53)           | (66) | (78) |
| Hardy [Member]                                                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (6)            |           | (6) |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             |      |           |    |   |
| Total pretax favorable unfavorable net prior year development, excluding Life & Group                                                  | (6)            |           | (6) |   |
| Corporate and Other Non-Core [Member]                                                                                                      |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | 3              | 11             | 1 | 5 |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (1)            | 1              | 1 |    |
| Total pretax favorable unfavorable net prior year development, excluding Life & Group                                                  | $ 2            | $ 12           | $ 2 | $ 5 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, P&C
+ References
No definition available.
+ Details**Name:**cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax Favorable Unfavorable Premium Development
+ References
No definition available.
+ Details**Name:**cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group
+ References
No definition available.
+ Details**Name:**cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |