| **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 | 120 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|------------------|------------------|
||
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total                                                                                                                                                                                                        | $ 47            | $ 30            | $ 9             |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Medical Professional Liability                                                                                                                                                                               |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | 47              | 58              | 60              |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total net development for accident years prior to 2009                                                                                                                                                       | 1               | (21)            | (51)            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total unallocated claim adjustment expense development                                                                                                                                                       | (1)             | (7)             | 0               |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total                                                                                                                                                                                                        | 47              | 30              | 9               |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Medical Professional Liability | 2009                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | 8               | (7)             | (27)            | $ (10)          | $ (20)          | $ (26)          | $ 0             | $ 2             | $ 1              |             |   |   |   |   |   |   |   | $ (79) |
| Medical Professional Liability | 2010                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | (4)             | (5)             | (30)            | (20)            | (11)            | 3               | 11              | $ 10            |             |             |   |   |   |   |   |   | $ (46) |   |
| Medical Professional Liability | 2011                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | 0               | 3               | (5)             | (29)            | 25              | 6               | $ 8             |            |             |             |   |   |   |   |   | $ 8 |   |   |
| Medical Professional Liability | 2012                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | 9               | (9)             | (5)             | (10)            | 39              | $ 5             |            |            |             |             |   |   |   |   | $ 29 |   |   |   |
| Medical Professional Liability | 2013                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | 10              | 12              | 13              | 21              | $ 17            |            |            |            |             |             |   |   |   | $ 73 |   |   |   |   |
| Medical Professional Liability | 2014                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | 5               | (7)             | 48              | $ 39            |            |            |            |            |             |             |   |   | $ 85 |   |   |   |   |   |
| Medical Professional Liability | 2015                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | (16)            | 11              | $ 66            |            |            |            |            |            |             |             |   | $ 61 |   |   |   |   |   |   |
| Medical Professional Liability | 2016                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | (2)             | $ 60            |            |            |            |            |            |            |             |             | $ 58 |   |   |   |   |   |   |   |
| Medical Professional Liability | 2017                                                                                                                                                                        |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                          |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                  | $ 37            |            |            |            |            |            |            |            |             | $ 37             |   |   |   |   |   |   |   |   |

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