| **Claim and Claim Adjustment Expense Reserves and Future Policy Benefit Reserves (Net prior year development) (Details) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Segment Reporting Information [Line Items]**                                                                                                    |           |           |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                                    | $ (17)         | $ (10)         | $ (2) | $ 4 |
| Specialty                                                                                                                                         |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                                    |           |           |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                                    | (15)           | (15)           | (35) | (40) |
| Commercial                                                                                                                                        |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                                    |           |           |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                                    | (2)            | 2              | (26) | 2 |
| International                                                                                                                                     |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                                    |           |           |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                                    | 0              | 3              | (5) | 2 |
| Corporate & Other                                                                                                                             |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                                    |           |           |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                                    | $ 0            | $ 0            | $ 64 | $ 40 |

| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax Favorable Unfavorable Premium Development, excluding Life & Group
+ References
No definition available.
+ Details**Name:**cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup**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_SegmentReportingInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtySegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CommercialSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_InternationalSegmentMember**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:** |