| **Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - 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, Workers' Compensation                                                                                               | $ (13)          | $ (32)          | $ (63)          |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Workers' Compensation                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                          | (60)            | (33)            | (100)           |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Adjustment for development on a discounted basis                                                                                                                                                     | 3               | 0               | (3)             |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total net development for accident years prior to 2010                                                                                                                                               | 21              | 8               | 39              |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total unallocated claim adjustment expense development                                                                                                                                               | 23              | (7)             | 1               |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Net prior year claim and allocated claim adjustment expense reserve development, Workers' Compensation                                                                                               | (13)            | (32)            | (63)            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Workers' Compensation | 2010                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                          | 3               | (1)             | 3               | $ 20            | $ 12            | $ 22            | $ 22            | $ 22            | $ 49             |             |   |   |   |   |   |   |   | $ 152 |
| Workers' Compensation | 2011                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                          | 14              | (2)             | 0               | 25              | (8)             | 12              | 6               | $ 34            |             |             |   |   |   |   |   |   | $ 81 |   |
| Workers' Compensation | 2012                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                          | (3)             | (2)             | (5)             | 9               | 10              | 32              | $ 26            |            |             |             |   |   |   |   |   | $ 67 |   |   |
| Workers' Compensation | 2013                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                          | (21)            | (11)            | (25)            | 26              | 20              | $ 35            |            |            |             |             |   |   |   |   | $ 24 |   |   |   |
| Workers' Compensation | 2014                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                          | (4)             | (2)             | (27)            | (1)             | $ 13            |            |            |            |             |             |   |   |   | $ (21) |   |   |   |   |
| Workers' Compensation | 2015                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                          | (14)            | 2               | (25)            | $ 9             |            |            |            |            |             |             |   |   | $ (28) |   |   |   |   |   |
| Workers' Compensation | 2016                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                          | (14)            | (9)             | $ (21)          |            |            |            |            |            |             |             |   | $ (44) |   |   |   |   |   |   |
| Workers' Compensation | 2017                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                          | (11)            | $ (8)           |            |            |            |            |            |            |             |             | $ (19) |   |   |   |   |   |   |   |
| Workers' Compensation | 2018                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                          | $ (10)          |            |            |            |            |            |            |            |             | $ (10)           |   |   |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Adjustment for Development on a Discounted Basis
+ References
No definition available.
+ Details**Name:**cna_AdjustmentforDevelopmentonaDiscountedBasis**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, Workers' Compensation
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentWorkersCompensation**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 |
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