| **Reinsurance (Narrative) (Details) - USD ($) $ in Millions**                                                                        | 12 Months Ended |         |
|--------------------------------------------------------------------------------------------------------------------------------------|-----------------|---------|
||
| **Ceded Credit Risk [Line Items]**                                                                                                   |            |    |   |   |
| Funds held under reinsurance agreements, liability                                                                                   | $ 3,700         | $ 4,000 |   | $ 690 |
| Largest recoverable from single reinsurer                                                                                            | 5,416           | 5,463   |   |   |
| Reinsurance recoveries reported in insurance claims and policyholders' benefits reported on the Consolidated Statement of Operations | 2,631           | 3,058   | $ 3,158 |   |
| Subsidiaries of Berkshire Hathaway Group                                                                                             |            |    |   |   |
| **Ceded Credit Risk [Line Items]**                                                                                                   |            |    |   |   |
| Largest recoverable from single reinsurer                                                                                            | 1,900           |    |   |   |
| Cavello Bay Reinsurance Limited                                                                                                      |            |    |   |   |
| **Ceded Credit Risk [Line Items]**                                                                                                   |            |    |   |   |
| Largest recoverable from single reinsurer                                                                                            | 598             |    |   |   |
| Gateway Rivers Insurance Company                                                                                                     |            |    |   |   |
| **Ceded Credit Risk [Line Items]**                                                                                                   |            |    |   |   |
| Largest recoverable from single reinsurer                                                                                            | 446             |    |   |   |
| Significant Captive Program                                                                                                          |            |    |   |   |
| **Ceded Credit Risk [Line Items]**                                                                                                   |            |    |   |   |
| Direct and ceded earned premiums                                                                                                     | 3,270           | 3,638   | 3,543 |   |
| Reinsurance recoveries reported in insurance claims and policyholders' benefits reported on the Consolidated Statement of Operations | $ 1,796         | $ 2,003 | $ 2,375 |   |

| 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
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_CededCreditRiskLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of funds held as security for payments to be received under reinsurance contracts.
+ References
No definition available.
+ 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI https://asc.fasb.org/extlink&oid=99397103&loc=d3e6811-158387
+ Details**Name:**us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after valuation allowance, recoverable under reinsurance contracts including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(6)) -URI https://asc.fasb.org/extlink&oid=126734703&loc=d3e572229-122910Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI https://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI https://asc.fasb.org/extlink&oid=99404803&loc=d3e21409-158489
+ Details**Name:**us-gaap_ReinsuranceRecoverables**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CededCreditRiskAxis=cna_SubsidiariesOfBerkshireHathawayGroupMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CededCreditRiskAxis=cna_CavelloBayReinsuranceLimitedMember**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_SignificantCaptiveProgramMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |