| **Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Specialty - Other Professional Liability and Management Liability - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Specialty - USD ($) $ in Millions** | 12 Months Ended | 24 Months Ended | 36 Months Ended | 48 Months Ended | 60 Months Ended | 72 Months Ended | 84 Months Ended | 96 Months Ended | 108 Months Ended | 120 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|------------------|------------------|
||
| **Claims Development [Line Items]**                                                                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total                                                                                                                                                                                                                                                     | $ 24            | $ (15)          | $ (69)          |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Other Professional Liability and Management Liability                                                                                                                                                                                                     |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                                               | 34              | 6               | (35)            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total net development for accident years prior to 2012                                                                                                                                                                                                    | (12)            | (21)            | (20)            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total unallocated claim adjustment expense development                                                                                                                                                                                                    | 2               | 0               | (14)            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total                                                                                                                                                                                                                                                     | 24              | (15)            | (69)            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Other Professional Liability and Management Liability | 2012                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                                               | (2)             | 19              | (2)             | $ (13)          | $ 6             | $ (38)          | $ (9)           | $ (22)          | $ (14)           |             |   |   |   |   |   |   |   | $ (75) |
| Other Professional Liability and Management Liability | 2013                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                                               | (13)            | (4)             | (13)            | (3)             | (19)            | (41)            | 32              | $ 10            |             |             |   |   |   |   |   |   | $ (51) |   |
| Other Professional Liability and Management Liability | 2014                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                                               | (4)             | (9)             | 19              | 4               | (54)            | (13)            | $ 20            |            |             |             |   |   |   |   |   | $ (37) |   |   |
| Other Professional Liability and Management Liability | 2015                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                                               | 23              | 6               | (25)            | (45)            | (15)            | $ 4             |            |            |             |             |   |   |   |   | $ (52) |   |   |   |
| Other Professional Liability and Management Liability | 2016                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                                               | (16)            | 3               | 4               | 0               | $ (1)           |            |            |            |             |             |   |   |   | $ (10) |   |   |   |   |
| Other Professional Liability and Management Liability | 2017                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                                               | (16)            | (22)            | (32)            | $ (2)           |            |            |            |            |             |             |   |   | $ (72) |   |   |   |   |   |
| Other Professional Liability and Management Liability | 2018                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                                               | 37              | 5               | $ 14            |            |            |            |            |            |             |             |   | $ 56 |   |   |   |   |   |   |
| Other Professional Liability and Management Liability | 2019                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                                               | 11              | $ 8             |            |            |            |            |            |            |             |             | $ 19 |   |   |   |   |   |   |   |
| Other Professional Liability and Management Liability | 2020                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                                               | $ 14            |            |            |            |            |            |            |            |             | $ 14             |   |   |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopment**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development prior years
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentPriorYears**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability
+ References
No definition available.
+ Details**Name:**cna_NetprioryearclaimandallocatedclaimadjustmentexpensereservedevelopmentOtherProfessionalLiabilityandManagementLiability**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unallocated Claim Adjustment Expense Development
+ References
No definition available.
+ Details**Name:**cna_UnallocatedClaimAdjustmentExpenseDevelopment**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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.
+ Details**Name:**us-gaap_ClaimsDevelopmentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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