| **Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Strengthening or Releasing of Prior Accident Year Reserves Commercial - Workers Compensation (Details) - Commercial - Workers Compensation [Member] - 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 |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|-----------------|------------------|
||
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                            | $ 125           | $ 69            | $ 125           |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Discounted Liabilities, Aggregate Discount                                                                                                                                                                             | 1               | (4)             | 1               |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development Prior Years                                                                                                                                                                | 24              | 15              | 13              |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net prior year claim and allocated claim adjustment expense reserve development, Workers' Compensation                                                                                                                                                     | 150             | 80              | 139             |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2007 [Member]                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                            | 19              | (2)             | 6               | $ 1             | $ 0             | $ (1)           | $ 8             | $ 16            | $ 12             |   |   |   |   |   |   |   | $ 59 |
| Short-duration Insurance Contracts, Accident Year 2008 [Member]                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                            | 8               | 8               | 10              | 4               | 2               | 13              | 18              | $ 17            |             |   |   |   |   |   |   | $ 80 |   |
| Short-duration Insurance Contracts, Accident Year 2009 [Member]                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                            | 8               | 6               | 11              | 4               | 2               | 7               | $ 4             |            |             |   |   |   |   |   | $ 42 |   |   |
| Short-duration Insurance Contracts, Accident Year 2010 [Member]                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                            | 20              | 14              | 20              | 22              | 22              | $ 43            |            |            |             |   |   |   |   | $ 141 |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2011 [Member]                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                            | 24              | (6)             | 11              | 9               | $ 35            |            |            |            |             |   |   |   | $ 73 |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2012 [Member]                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                            | 10              | 13              | 32              | $ 27            |            |            |            |            |             |   |   | $ 82 |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2013 [Member]                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                            | 26              | 21              | $ 35            |            |            |            |            |            |             |   | $ 82 |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2014 [Member]                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                            | 0               | $ 15            |            |            |            |            |            |            |             | $ 15 |   |   |   |   |   |   |   |
| Short-duration Insurance Contracts, Accident Year 2015 [Member]                                                                                                                                                                                            |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                                                                                                                                     |            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |
| Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development                                                                                                                                                                            | $ 10            |            |            |            |            |            |            |            |             |   |   |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Adjustment for Development on a Discounted Basis
+ References
No definition available.
+ Details**Name:**cna_AdjustmentforDevelopmentonaDiscountedBasis**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, Workers' Compensation
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentWorkersCompensation**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
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_CommercialWorkersCompensationMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2007Member**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:** |