| **Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Specialty Reconciliation (Details) - USD ($) $ in Millions** | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 |
|---------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|---------------|
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                |          |          |          |          |
| Liability for Unpaid Claims and Claims Adjustment Expense, Net                                                                        | $ 18,249      | $ 18,576      | $ 18,927      | $ 19,117      |
| Specialty - Medical Professional Liability [Member]                                                                                   |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                |          |          |          |          |
| Liability for Unpaid Claims and Claims Adjustment Expense, Net                                                                        | 1,779         |          |          |          |
| Specialty - Other Professional and Management Liability [Member]                                                                      |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                |          |          |          |          |
| Liability for Unpaid Claims and Claims Adjustment Expense, Net                                                                        | 3,063         |          |          |          |
| Specialty - Surety [Member]                                                                                                           |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                |          |          |          |          |
| Liability for Unpaid Claims and Claims Adjustment Expense, Net                                                                        | 394           |          |          |          |
| Specialty - Warranty [Member]                                                                                                         |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                |          |          |          |          |
| Liability for Unpaid Claims and Claims Adjustment Expense, Net                                                                        | 37            |          |          |          |
| Specialty - Other [Member]                                                                                                            |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                |          |          |          |          |
| Liability for Unpaid Claims and Claims Adjustment Expense, Net                                                                        | 131           |          |          |          |
| Specialty [Member]                                                                                                                    |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                |          |          |          |          |
| Liability for Unpaid Claims and Claims Adjustment Expense, Net                                                                        | $ 5,404       | $ 5,568       |          |          |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(13)(a)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437
+ Details**Name:**us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| 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_SpecialtyMedicalProfessionalLiabilityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtyOtherProfessionalandManagementLiabilityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtySuretyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtyWarrantyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtyOtherMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |