| **Claim, Claim Adjustment Expense and Future Policy Benefit 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                                                                                                                                                                                                                              | $ 23            | $ 35            | $ 75            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Medical Professional Liability                                                                                                                                                                                                     |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | 13              | 32              | 52              |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total net development for accident years prior to 2012                                                                                                                                                                             | 3               | 3               | 21              |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total unallocated claim adjustment expense development                                                                                                                                                                             | 7               | 0               | 2               |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Total                                                                                                                                                                                                                              | 23              | 35              | 75              |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Medical Professional Liability | 2012                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | 0               | (2)             | 6               | $ 9             | $ (9)           | $ (5)           | $ (10)          | $ 39            | $ 5              |             |   |   |   |   |   |   |   | $ 33 |
| Medical Professional Liability | 2013                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | (1)             | (14)            | 10              | 10              | 12              | 13              | 21              | $ 17            |             |             |   |   |   |   |   |   | $ 68 |   |
| Medical Professional Liability | 2014                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | (3)             | (2)             | (6)             | 5               | (7)             | 48              | $ 39            |            |             |             |   |   |   |   |   | $ 74 |   |   |
| Medical Professional Liability | 2015                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | (9)             | 22              | (6)             | (16)            | 11              | $ 66            |            |            |             |             |   |   |   |   | $ 68 |   |   |   |
| Medical Professional Liability | 2016                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | 2               | 9               | 14              | (2)             | $ 60            |            |            |            |             |             |   |   |   | $ 83 |   |   |   |   |
| Medical Professional Liability | 2017                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | (5)             | 2               | 9               | $ 37            |            |            |            |            |             |             |   |   | $ 43 |   |   |   |   |   |
| Medical Professional Liability | 2018                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | 17              | 2               | $ 25            |            |            |            |            |            |             |             |   | $ 44 |   |   |   |   |   |   |
| Medical Professional Liability | 2019                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | 13              | $ 15            |            |            |            |            |            |            |             |             | $ 28 |   |   |   |   |   |   |   |
| Medical Professional Liability | 2020                                                                                                                                                                                              |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| **Claims Development [Line Items]**                                                                                                                                                                                                |            |            |            |            |            |            |            |            |             |             |   |   |   |   |   |   |   |   |
| Cumulative Number of Claims                                                                                                                                                                                                        | $ (1)           |            |            |            |            |            |            |            |             | $ (1)            |   |   |   |   |   |   |   |   |

| 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_SpecialtySegmentMember**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_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:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortDurationInsuranceContractsAccidentYear2018Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortDurationInsuranceContractAccidentYear2019Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortDurationInsuranceContractAccidentYear2020Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |