| **Claim and Claim Adjustment Expense 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]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability                                                                                             | $ (69)          | $ (127)         | $ (126)         |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Other Professional Liability and Management Liability                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | (38)            | (76)            | (92)            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total net development for accident years prior to 2010                                                                                                                                                                             | (17)            | (44)            | (27)            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total unallocated claim adjustment expense development                                                                                                                                                                             | (14)            | (7)             | (7)             |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability                                                                                             | (69)            | (127)           | (126)           |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Other Professional Liability and Management Liability | 2010                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | 0               | (6)             | 3               | $ (13)          | $ (10)          | $ (1)           | $ 0             | $ 20            | $ 0              |             |   |   |   |   |   |   |   | $ (7) |
| Other Professional Liability and Management Liability | 2011                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | (3)             | (11)            | (12)            | (33)            | (5)             | 15              | 26              | $ 28            |             |             |   |   |   |   |   |   | $ 5 |   |
| Other Professional Liability and Management Liability | 2012                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | (2)             | (13)            | 6               | (38)            | (9)             | (22)            | $ (14)          |            |             |             |   |   |   |   |   | $ (92) |   |   |
| Other Professional Liability and Management Liability | 2013                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | (13)            | (3)             | (19)            | (41)            | 32              | $ 10            |            |            |             |             |   |   |   |   | $ (34) |   |   |   |
| Other Professional Liability and Management Liability | 2014                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | 19              | 4               | (54)            | (13)            | $ 20            |            |            |            |             |             |   |   |   | $ (24) |   |   |   |   |
| Other Professional Liability and Management Liability | 2015                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | (25)            | (45)            | (15)            | $ 4             |            |            |            |            |             |             |   |   | $ (81) |   |   |   |   |   |
| Other Professional Liability and Management Liability | 2016                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | 4               | 0               | $ (1)           |            |            |            |            |            |             |             |   | $ 3 |   |   |   |   |   |   |
| Other Professional Liability and Management Liability | 2017                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | (32)            | $ (2)           |            |            |            |            |            |            |             |             | $ (34) |   |   |   |   |   |   |   |
| Other Professional Liability and Management Liability | 2018                                                                                                                                                                       |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **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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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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