| **Claim and Claim Adjustment Expense 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]**                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto                                                                                               | $ (25)          | $ 1             | $ (35)          |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Commercial Auto                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                    | (20)            | (1)             | (41)            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total net development for accident years prior to 2010                                                                                                                                         | (4)             | 1               | 4               |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total unallocated claim adjustment expense development                                                                                                                                         | (1)             | 1               | 2               |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto                                                                                               | (25)            | 1               | (35)            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| 2010 | Commercial Auto                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                    | 0               | 0               | (1)             | $ (4)           | $ (5)           | $ 7             | $ 4             | $ 4             | $ 16             |             |   |   |   |   |   |   |   | $ 21 |
| 2011 | Commercial Auto                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                    | (3)             | 0               | 0               | (6)             | (2)             | 14              | 7               | $ 13            |             |             |   |   |   |   |   |   | $ 23 |   |
| 2012 | Commercial Auto                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                    | (2)             | 0               | (8)             | 4               | 4               | 10              | $ 14            |            |             |             |   |   |   |   |   | $ 22 |   |   |
| 2013 | Commercial Auto                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                    | (4)             | 0               | (4)             | (16)            | 0               | $ 19            |            |            |             |             |   |   |   |   | $ (5) |   |   |   |
| 2014 | Commercial Auto                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                    | (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                                                                                                                                                                    | (7)             | 0               | (9)             | $ (2)           |            |            |            |            |             |             |   |   | $ (18) |   |   |   |   |   |
| 2016 | Commercial Auto                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                    | 0               | 0               | $ (12)          |            |            |            |            |            |             |             |   | $ (12) |   |   |   |   |   |   |
| 2017 | Commercial Auto                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                    | 2               | $ (1)           |            |            |            |            |            |            |             |             | $ 1 |   |   |   |   |   |   |   |
| 2018 | Commercial Auto                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                    | $ (2)           |            |            |            |            |            |            |            |             | $ (2)            |   |   |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto
+ References
No definition available.
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| 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 |
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