| **Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Consolidated 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      |
| Reinsurance Recoverable for Unpaid Claims and Claims Adjustments                                                                         | 4,094         |          |          |          |
| Total Gross Carried Claim and Claim Adjustment Expense Reserves                                                                          | 22,343        | 22,663        | $ 23,271      | $ 24,089      |
| 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         |          |          |
| Reinsurance Recoverable for Unpaid Claims and Claims Adjustments                                                                         | 745           |          |          |          |
| Total Gross Carried Claim and Claim Adjustment Expense Reserves                                                                          | 6,149         | 6,269         |          |          |
| Commercial [Member]                                                                                                                      |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                   |          |          |          |          |
| Liability for Unpaid Claims and Claims Adjustment Expense, Net                                                                           | 8,305         | 8,576         |          |          |
| Reinsurance Recoverable for Unpaid Claims and Claims Adjustments                                                                         | 589           |          |          |          |
| Total Gross Carried Claim and Claim Adjustment Expense Reserves                                                                          | 8,894         | 9,183         |          |          |
| International [Member]                                                                                                                   |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                   |          |          |          |          |
| Liability for Unpaid Claims and Claims Adjustment Expense, Net                                                                           | 1,201         | 1,219         |          |          |
| Reinsurance Recoverable for Unpaid Claims and Claims Adjustments                                                                         | 127           |          |          |          |
| Total Gross Carried Claim and Claim Adjustment Expense Reserves                                                                          | 1,328         | 1,347         |          |          |
| Corporate and Other [Member]                                                                                                             |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                   |          |          |          |          |
| Liability for Unpaid Claims and Claims Adjustment Expense, Net                                                                           | 230           | 283           |          |          |
| Reinsurance Recoverable for Unpaid Claims and Claims Adjustments                                                                         | 2,384         |          |          |          |
| Total Gross Carried Claim and Claim Adjustment Expense Reserves                                                                          | 2,614         | 2,644         |          |          |
| Life and Group Non-Core [Member]                                                                                                         |          |          |          |          |
| **Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]**                                   |          |          |          |          |
| Liability for Unpaid Claims and Claims Adjustment Expense, Net                                                                           | 3,109         | 2,930         |          |          |
| Reinsurance Recoverable for Unpaid Claims and Claims Adjustments                                                                         | 249           |          |          |          |
| Total Gross Carried Claim and Claim Adjustment Expense Reserves                                                                          | $ 3,358       | $ 3,220       |          |          |

| X                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Liability for unpaid claim and claim adjustment expense, gross
+ References
No definition available.
+ Details**Name:**cna_Liabilityforunpaidclaimandclaimadjustmentexpensegross**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| 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
Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.
+ References
Reference 1: 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 2: 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=d3e14764-158437Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489
+ Details**Name:**us-gaap_ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**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_SpecialtyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CommercialMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_InternationalMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateAndOtherMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_LifeAndGroupNonCoreMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |