| **Claim and Claim Adjustment Expense Reserves (Specialty - Medical Professional Liability -  Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Specialty - 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 |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|------------------|
||
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Total                                                                                                                                                                                                        | $ 5             | $ (37)          | $ (43)          |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Medical Professional Liability                                                                                                                                                                               |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | 32              | (2)             | (27)            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Total net development for accident years prior to 2008                                                                                                                                                       | (19)            | (35)            | (16)            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Total unallocated claim adjustment expense development                                                                                                                                                       | (8)             | 0               | 0               |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Total                                                                                                                                                                                                        | 5               | (37)            | (43)            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Medical Professional Liability | 2008                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | (4)             | (13)            | (12)            | $ (1)           | $ 0             | $ (12)          | $ (16)          | $ 45            | $ 25             |   |   |   |   |   |   |   | $ 12 |
| Medical Professional Liability | 2009                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | (10)            | (31)            | (8)             | (9)             | (26)            | 12              | 25              | $ 7             |             |   |   |   |   |   |   | $ (40) |   |
| Medical Professional Liability | 2010                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | (5)             | (43)            | (24)            | (16)            | 8               | 0               | $ (5)           |            |             |   |   |   |   |   | $ (85) |   |   |
| Medical Professional Liability | 2011                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | 0               | (10)            | (32)            | 26              | 15              | $ 6             |            |            |             |   |   |   |   | $ 5 |   |   |   |
| Medical Professional Liability | 2012                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | 4               | (8)             | (8)             | 46              | $ 3             |            |            |            |             |   |   |   | $ 37 |   |   |   |   |
| Medical Professional Liability | 2013                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | 6               | 7               | 20              | $ 6             |            |            |            |            |             |   |   | $ 39 |   |   |   |   |   |
| Medical Professional Liability | 2014                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | (21)            | 37              | $ 37            |            |            |            |            |            |             |   | $ 53 |   |   |   |   |   |   |
| Medical Professional Liability | 2015                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | 4               | 59              |            |            |            |            |            |            |             | $ 63 |   |   |   |   |   |   |   |
| Medical Professional Liability | 2016                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | $ 58            | $ 58            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Medical Professional Liability
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentMedicalProfessionalLiability**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_ClaimsDevelopmentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ProductOrServiceAxis=cna_MedicalProfessionalLiabilityMember**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:** |