| **Claim and Claim Adjustment Expense Reserves Short Duration Contracts – Net Strengthening or Releasing of Prior Accident Year Reserves - International - Hardy (Details) - Hardy [Member] - USD ($) $ in Millions** | 12 Months Ended | 24 Months Ended | 36 Months Ended | 48 Months Ended | 60 Months Ended |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|-----------------|-----------------|-----------------|
||
| 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                                                                                                                                            | $ 5             | $ (6)           | $ 5             | $ (1)           | $ 7             |   |   |   | $ 10 |
| 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                                                                                                                                            | 0               | 1               | 1               | (3)             | (4)             |   |   |   | (5) |
| 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                                                                                                                                            | (3)             | 5               | (10)            | (6)             | 1               |   |   |   | (13) |
| 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                                                                                                                                            | 6               | (6)             | 4               | 10              | (10)            |   |   |   | 4 |
| 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                                                                                                                                            | (2)             | 4               | 0               | 9               | $ 0             |   |   |   | $ 11 |
| 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                                                                                                                                            | (7)             | 7               | 8               | $ 0             |            |   |   | $ 8 |   |
| 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                                                                                                                                            | 1               | (8)             | $ 17            |            |            |   | $ 10 |   |   |
| 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                                                                                                                                            | (6)             | $ (2)           |            |            |            | $ (8) |   |   |   |
| 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                                                                                                                                            | $ (11)          |            |            |            |            |   |   |   |   |

| X                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopment**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsReconciliationOfClaimsDevelopmentToLiabilityLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_HardyMember**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:** |