| **Claim and Claim Adjustment Expense Reserves (International - Excluding Hardy - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Excluding Hardy - International - 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 |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|------------------|
||
| 2008                                                                                                                                                                                                                   |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                            | $ (1)           | $ (4)           | $ 2             | $ 9             | $ (8)           | $ 0             | $ (8)           | $ (12)          | $ 3              |   |   |   |   |   |   |   | $ (19) |
| 2009                                                                                                                                                                                                                   |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                            | (1)             | (9)             | (10)            | (4)             | (15)            | 1               | 2               | $ (4)           |             |   |   |   |   |   |   | $ (40) |   |
| 2010                                                                                                                                                                                                                   |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                            | (6)             | (7)             | (7)             | (10)            | (7)             | (5)             | $ (3)           |            |             |   |   |   |   |   | $ (45) |   |   |
| 2011                                                                                                                                                                                                                   |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                            | (3)             | (7)             | (11)            | (22)            | (8)             | $ 2             |            |            |             |   |   |   |   | $ (49) |   |   |   |
| 2012                                                                                                                                                                                                                   |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                            | (8)             | 1               | (7)             | (16)            | $ 7             |            |            |            |             |   |   |   | $ (23) |   |   |   |   |
| 2013                                                                                                                                                                                                                   |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                            | (5)             | (20)            | (9)             | $ 3             |            |            |            |            |             |   |   | $ (31) |   |   |   |   |   |
| 2014                                                                                                                                                                                                                   |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                            | (12)            | 0               | $ 16            |            |            |            |            |            |             |   | $ 4 |   |   |   |   |   |   |
| 2015                                                                                                                                                                                                                   |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                            | (2)             | 16              |            |            |            |            |            |            |             | $ 14 |   |   |   |   |   |   |   |
| 2016                                                                                                                                                                                                                   |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                 |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                            | $ 19            | $ 19            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |

| 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
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_ProductOrServiceAxis=cna_ExcludingHardyMember**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_StatementBusinessSegmentsAxis=cna_InternationalMember**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:** |