| **Claim and Claim Adjustment Expense Reserves (International - Hardy - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - International - Hardy - 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 |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|------------------|------------------|
||
| 2009                                                                                                                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                             |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                        | $ 0             | $ (1)           | $ (3)           | $ 5             | $ (10)          | $ (6)           | $ 1             |            |             |             |   |   |   |   |   | $ (14) |
| 2010                                                                                                                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                             |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                        | (1)             | (1)             | 6               | (6)             | 5               | 9               | (10)            |            |             |             |   |   |   |   | $ 2 |   |
| 2011                                                                                                                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                             |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                        | 3               | 0               | (1)             | 4               | 0               | 10              | $ (1)           |            |             |             |   |   |   | $ 15 |   |   |
| 2012                                                                                                                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                             |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                        | 2               | 0               | (6)             | 7               | 8               | $ 0             |            |            |             |             |   |   | $ 11 |   |   |   |
| 2013                                                                                                                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                             |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                        | 4               | 1               | 1               | (8)             | $ 16            |            |            |            |             |             |   | $ 14 |   |   |   |   |
| 2014                                                                                                                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                             |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                        | 0               | (6)             | (6)             | $ (2)           |            |            |            |            |             |             | $ (14) |   |   |   |   |   |
| 2015                                                                                                                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                             |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                        | 1               | (2)             | $ (11)          |            |            |            |            |            |             | $ (12)           |   |   |   |   |   |   |
| 2016                                                                                                                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                             |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                        | (11)            | $ 18            |            |            |            |            |            |            | $ 7              |             |   |   |   |   |   |   |
| 2017                                                                                                                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                             |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                        | $ 8             |            |            |            |            |            |            | $ 8             |             |             |   |   |   |   |   |   |

| 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_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:**srt_ProductOrServiceAxis=cna_HardyMember**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:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortDurationInsuranceContractsAccidentYear2017Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |