| **Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $)In Millions, unless otherwise specified** | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:**                   |            |   |   |
| Core (Non-A&EP)                                                                                                                   |            |   | $ (444) |
| A&EP                                                                                                                              |            |   | 155 |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, P&C               | (423)           | (637) | (289) |
| Pretax (favorable) unfavorable premium development                                                                                    | (8)             | 43 | 81 |
| Total pretax (favorable) unfavorable net prior year development                                                                       | (431)           | (594) | (208) |
| CNA Specialty [Member]                                                                                                                |            |   |   |
| **Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:**                   |            |   |   |
| Core (Non-A&EP)                                                                                                                   |            |   | (218) |
| A&EP                                                                                                                              |            |   |    |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, P&C               | (217)           | (341) | (218) |
| Pretax (favorable) unfavorable premium development                                                                                    | (28)            | (3) | (6) |
| Total pretax (favorable) unfavorable net prior year development                                                                       | (245)           | (344) | (224) |
| CNA Commercial [Member]                                                                                                               |            |   |   |
| **Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:**                   |            |   |   |
| Core (Non-A&EP)                                                                                                                   |            |   | (230) |
| A&EP                                                                                                                              |            |   |    |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, P&C               | (204)           | (304) | (230) |
| Pretax (favorable) unfavorable premium development                                                                                    | 21              | 48 | 87 |
| Total pretax (favorable) unfavorable net prior year development                                                                       | (183)           | (256) | (143) |
| Corporate and Other Non-Core [Member]                                                                                                 |            |   |   |
| **Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:**                   |            |   |   |
| Core (Non-A&EP)                                                                                                                   |            |   | 4 |
| A&EP                                                                                                                              |            |   | 155 |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, P&C               | (2)             | 8 | 159 |
| Pretax (favorable) unfavorable premium development                                                                                    | (1)             | (2) |    |
| Total pretax (favorable) unfavorable net prior year development                                                                       | $ (3)           | $ 6 | $ 159 |

| X                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Liability for other than asbestos and environmental claims net incurred loss
+ References
No definition available.
+ Details**Name:**cna_LiabilityForOtherThanAsbestosAndEnvironmentalClaimsNetIncurredLoss**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentAbstract**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| 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_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentPc**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_PretaxFavorableUnfavorablePremiumDevelopment**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total Pretax Favorable Unfavorable Net Prior Year Development
+ References
No definition available.
+ Details**Name:**cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopment**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The increase (decrease) during the reporting period in asbestos and environmental claims reserve, net of reinsurance recoveries.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b),(c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437
+ Details**Name:**us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetPeriodIncreaseDecrease**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**duration |