| **Claim and Claim Adjustment Expense Reserves (Commercial - General Liability - 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 |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|------------------|
||
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                           |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net prior year claim and allocated claim adjustment expense reserve development, General Liability                                                                                               | $ 0             | $ (28)          | $ (33)          |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| General Liability                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                           |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                      | (5)             | (13)            | (49)            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Total net development for accident years prior to 2008                                                                                                                                           | (2)             | (15)            | 16              |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Total unallocated claim adjustment expense development                                                                                                                                           | 7               | 0               | 0               |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net prior year claim and allocated claim adjustment expense reserve development, General Liability                                                                                               | 0               | (28)            | (33)            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| General Liability | 2008                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                           |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                      | 1               | (9)             | (13)            | $ (19)          | $ (1)           | $ (14)          | $ 17            | $ 26            | $ (7)            |   |   |   |   |   |   |   | $ (19) |
| General Liability | 2009                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                           |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                      | 5               | 3               | (4)             | (6)             | (4)             | (1)             | (3)             | $ 46            |             |   |   |   |   |   |   | $ 36 |   |
| General Liability | 2010                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                           |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                      | (5)             | (16)            | (20)            | 46              | 50              | 2               | $ 31            |            |             |   |   |   |   |   | $ 88 |   |   |
| General Liability | 2011                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                           |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                      | (6)             | 10              | 1               | 46              | 30              | $ (3)           |            |            |             |   |   |   |   | $ 78 |   |   |   |
| General Liability | 2012                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                           |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                      | 11              | (12)            | (9)             | 16              | $ 24            |            |            |            |             |   |   |   | $ 30 |   |   |   |   |
| General Liability | 2013                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                           |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                      | (39)            | 9               | (11)            | $ 30            |            |            |            |            |             |   |   | $ (11) |   |   |   |   |   |
| General Liability | 2014                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                           |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                      | (8)             | 1               | $ 7             |            |            |            |            |            |             |   | $ 0 |   |   |   |   |   |   |
| General Liability | 2015                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                           |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                      | 11              | 1               |            |            |            |            |            |            |             | $ 12 |   |   |   |   |   |   |   |
| General Liability | 2016                                                                                                                                                                         |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                           |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                      | $ 25            | $ 25            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, General Liability
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentGeneralLiability**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                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CommercialMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ProductOrServiceAxis=us-gaap_GeneralLiabilityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2008Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2009Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2010Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2011Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2012Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2013Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2014Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2015Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2016Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |