| **Benefit Plans (Actuarial Assumptions for Benefit Obligations) (Details)** | Dec. 31, 2017 | Dec. 31, 2016 |
|-----------------------------------------------------------------------------|---------------|---------------|
| Pension Plans, Defined Benefit                                              |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                            |          |          |
| Discount rate                                                               | 3.55%         | 3.95%         |
| Expected long term rate of return                                           | 7.50%         | 7.50%         |
| Other Postretirement Benefits Plan                                          |          |          |
| **Defined Benefit Plan Disclosure [Line Items]**                            |          |          |
| Discount rate                                                               | 2.75%         | 2.75%         |

| X                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Long term rate of return on asset assumption used in calculating benefit plan obligation.
+ References
No definition available.
+ Details**Name:**cna_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationLongTermRateOfReturn**Namespace Prefix:**cna_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920
+ Details**Name:**us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**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_DefinedBenefitPlanDisclosureLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |