| **Reinsurance (Narrative) (Details) - USD ($) $ in Millions**                                                                        | 12 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| Funds held under reinsurance agreements, liability                                                                                   | $ 3,000         | $ 3,200 |   |
| Reinsurance recoveries reported in insurance claims and policyholders' benefits reported on the Consolidated Statement of Operations | 3,016           | 2,601 | $ 1,379 |
| Subsidiaries of Berkshire Hathaway Group [Member]                                                                                    |            |   |   |
| Largest Recoverables From Single Reinsurer                                                                                           | 2,353           |   |   |
| Gateway Rivers Insurance Company [Member]                                                                                            |            |   |   |
| Largest Recoverables From Single Reinsurer                                                                                           | 350             |   |   |
| Subsidiaries of Wilton Re[Member]                                                                                                    |            |   |   |
| Largest Recoverables From Single Reinsurer                                                                                           | 212             |   |   |
| Significant Captive Program [Member]                                                                                                 |            |   |   |
| Direct and ceded earned premiums                                                                                                     | 3,865           | 3,344 | 2,643 |
| Reinsurance recoveries reported in insurance claims and policyholders' benefits reported on the Consolidated Statement of Operations | $ 2,621         | $ 2,282 | $ 1,458 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Direct and ceded earned premiums related to business that is one hundred percent reinsured as a result of a significant captive program
+ References
No definition available.
+ Details**Name:**cna_DirectAndCededEarnedPremiumsRelatedToBusinessThatIsOneHundredPercentReinsuredAsResultOfSignificantCaptiveProgram**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Largest Recoverables from Single Reinsurer
+ References
No definition available.
+ Details**Name:**cna_LargestRecoverablesFromSingleReinsurer**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of funds held as security for payments to be received under reinsurance contracts.
+ 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(c)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910
+ Details**Name:**us-gaap_FundsHeldUnderReinsuranceAgreementsLiability**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of reduction of provision for policy benefits and costs incurred for policies ceded.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387
+ Details**Name:**us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CededCreditRiskAxis=cna_SubsidiariesOfBerkshireHathawayGroupMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CededCreditRiskAxis=cna_GatewayRiversInsuranceCompanyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CededCreditRiskAxis=cna_SubsidiariesofWiltonReMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CededCreditRiskAxis=cna_SignificantCaptiveProgramMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |