| **Claim and Claim Adjustment Expense Reserves and Future Policy Benefit Reserves (Commercial - Commercial Auto - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Commercial - 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 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|------------------|------------------|
||
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total                                                                                                                                                                                                                             | $ 49            | $ 53            | $ 33            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Commercial Auto                                                                                                                                                                                                                   |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                       | 47              | 53              | 32              |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total net development for accident years prior to 2013                                                                                                                                                                            | 2               | 0               | 1               |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total unallocated claim adjustment expense development                                                                                                                                                                            | 0               | 0               | 0               |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total                                                                                                                                                                                                                             | 49              | 53              | 33              |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| 2013 | Commercial Auto                                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                       | 2               | 0               | 0               | $ (4)           | $ 0             | $ (4)           | $ (16)          | $ 0             | $ 19             |             |   |   |   |   |   |   |   | $ (3) |
| 2014 | Commercial Auto                                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                       | (1)             | 1               | 0               | (4)             | 0               | (7)             | (11)            | $ (11)          |             |             |   |   |   |   |   |   | $ (33) |   |
| 2015 | Commercial Auto                                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                       | (1)             | 2               | (2)             | (7)             | 0               | (9)             | $ (2)           |            |             |             |   |   |   |   |   | $ (19) |   |   |
| 2016 | Commercial Auto                                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                       | 5               | 5               | 4               | 0               | 0               | $ (12)          |            |            |             |             |   |   |   |   | $ 2 |   |   |   |
| 2017 | Commercial Auto                                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                       | 7               | 11              | 21              | 2               | $ (1)           |            |            |            |             |             |   |   |   | $ 40 |   |   |   |   |
| 2018 | Commercial Auto                                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                       | 9               | 18              | 0               | $ (2)           |            |            |            |            |             |             |   |   | $ 25 |   |   |   |   |   |
| 2019 | Commercial Auto                                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                       | 34              | 23              | $ 9             |            |            |            |            |            |             |             |   | $ 66 |   |   |   |   |   |   |
| 2020 | Commercial Auto                                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                       | 1               | $ (7)           |            |            |            |            |            |            |             |             | $ (6) |   |   |   |   |   |   |   |
| 2022 | Commercial Auto                                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                            |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                       | $ (9)           |            |            |            |            |            |            |            |             | $ (9)            |   |   |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentCommercialAuto**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
+ 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
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_ShortdurationInsuranceContractsReconciliationOfClaimsDevelopmentToLiabilityLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2015Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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