| **Claim and Claim Adjustment Expense Reserves (Gross and net carried claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions** | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|---------------|
| Gross Case Reserves                                                                                                                                   | $ 11,860      | $ 12,102      |          |          |
| Gross IBNR Reserves                                                                                                                                   | 10,483        | 10,561        |          |          |
| Total Gross Carried Claim and Claim Adjustment Expense Reserves                                                                                       | 22,343        | 22,663        | $ 23,271      | $ 24,089      |
| Net Case Reserves                                                                                                                                     | 9,627         | 9,836         |          |          |
| Net IBNR Reserves                                                                                                                                     | 8,622         | 8,740         |          |          |
| Total Net Carried Claim and Claim Adjustment Expense Reserves                                                                                         | 18,249        | 18,576        | $ 18,927      | $ 19,117      |
| Specialty [Member]                                                                                                                                    |          |          |          |          |
| Gross Case Reserves                                                                                                                                   | 1,871         | 2,011         |          |          |
| Gross IBNR Reserves                                                                                                                                   | 4,278         | 4,258         |          |          |
| Total Gross Carried Claim and Claim Adjustment Expense Reserves                                                                                       | 6,149         | 6,269         |          |          |
| Net Case Reserves                                                                                                                                     | 1,681         | 1,810         |          |          |
| Net IBNR Reserves                                                                                                                                     | 3,723         | 3,758         |          |          |
| Total Net Carried Claim and Claim Adjustment Expense Reserves                                                                                         | 5,404         | 5,568         |          |          |
| Commercial [Member]                                                                                                                                   |          |          |          |          |
| Gross Case Reserves                                                                                                                                   | 4,661         | 4,975         |          |          |
| Gross IBNR Reserves                                                                                                                                   | 4,233         | 4,208         |          |          |
| Total Gross Carried Claim and Claim Adjustment Expense Reserves                                                                                       | 8,894         | 9,183         |          |          |
| Net Case Reserves                                                                                                                                     | 4,353         | 4,651         |          |          |
| Net IBNR Reserves                                                                                                                                     | 3,952         | 3,925         |          |          |
| Total Net Carried Claim and Claim Adjustment Expense Reserves                                                                                         | 8,305         | 8,576         |          |          |
| International [Member]                                                                                                                                |          |          |          |          |
| Gross Case Reserves                                                                                                                                   | 632           | 622           |          |          |
| Gross IBNR Reserves                                                                                                                                   | 696           | 725           |          |          |
| Total Gross Carried Claim and Claim Adjustment Expense Reserves                                                                                       | 1,328         | 1,347         |          |          |
| Net Case Reserves                                                                                                                                     | 548           | 531           |          |          |
| Net IBNR Reserves                                                                                                                                     | 653           | 688           |          |          |
| Total Net Carried Claim and Claim Adjustment Expense Reserves                                                                                         | 1,201         | 1,219         |          |          |
| Life and Group Non-Core [Member]                                                                                                                      |          |          |          |          |
| Gross Case Reserves                                                                                                                                   | 3,172         | 2,973         |          |          |
| Gross IBNR Reserves                                                                                                                                   | 186           | 247           |          |          |
| Total Gross Carried Claim and Claim Adjustment Expense Reserves                                                                                       | 3,358         | 3,220         |          |          |
| Net Case Reserves                                                                                                                                     | 2,951         | 2,714         |          |          |
| Net IBNR Reserves                                                                                                                                     | 158           | 216           |          |          |
| Total Net Carried Claim and Claim Adjustment Expense Reserves                                                                                         | 3,109         | 2,930         |          |          |
| Corporate and Other [Member]                                                                                                                          |          |          |          |          |
| Gross Case Reserves                                                                                                                                   | 1,524         | 1,521         |          |          |
| Gross IBNR Reserves                                                                                                                                   | 1,090         | 1,123         |          |          |
| Total Gross Carried Claim and Claim Adjustment Expense Reserves                                                                                       | 2,614         | 2,644         |          |          |
| Net Case Reserves                                                                                                                                     | 94            | 130           |          |          |
| Net IBNR Reserves                                                                                                                                     | 136           | 153           |          |          |
| Total Net Carried Claim and Claim Adjustment Expense Reserves                                                                                         | $ 230         | $ 283         |          |          |

| 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
Net case reserves
+ References
No definition available.
+ Details**Name:**cna_NetCaseReserves**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net IBNR reserves
+ References
No definition available.
+ Details**Name:**cna_NetIbnrReserves**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Estimated costs as of the balance sheet date of settling insured claims and costs incurred in the claims settlement process for claims that have not yet been submitted to the insurance company for reimbursement.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.W) -URI http://asc.fasb.org/extlink&oid=27015833&loc=d3e570705-122907
+ Details**Name:**us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsAmount**Namespace Prefix:**us-gaap_**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
Costs of settling insured claims and costs incurred in the claims settlement process for the specified year specifically for claims that have already been submitted to the insurance company for reimbursement.
+ References
No definition available.
+ Details**Name:**us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseReportedClaimsAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                             |
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