| **Cover Page Cover Page**                   | Sep. 18, 2024                 |
|---------------------------------------------|-------------------------------|
| **Entity Information [Line Items]**         |                          |
| Document Type                               | 8-K                           |
| Document Period End Date                    | Sep. 18,  2024                |
| Entity Registrant Name                      | CNA FINANCIAL CORPORATION     |
| Entity Incorporation, State or Country Code | DE                            |
| Entity File Number                          | 1-5823                        |
| Entity Tax Identification Number            | 36-6169860                    |
| Entity Address, Address Line One            | 151 N. Franklin               |
| Entity Address, City or Town                | Chicago                       |
| Entity Address, State or Province           | IL                            |
| Entity Address, Postal Zip Code             | 60606                         |
| City Area Code                              | 312                           |
| Local Phone Number                          | 822-5000                      |
| Written Communications                      | false                         |
| Soliciting Material                         | false                         |
| Pre-commencement Tender Offer               | false                         |
| Pre-commencement Issuer Tender Offer        | false                         |
| Entity Emerging Growth Company              | false                         |
| Entity Central Index Key                    | 0000021175                    |
| Amendment Flag                              | false                         |
| New York Stock Exchange                     |                          |
| **Entity Information [Line Items]**         |                          |
| Title of 12(b) Security                     | Common Stock, Par value $2.50 |
| Trading Symbol                              | "CNA"                         |
| Security Exchange Name                      | NYSE                          |
| Chicago Stock Exchange                      |                          |
| **Entity Information [Line Items]**         |                          |
| Title of 12(b) Security                     | Common Stock, Par value $2.50 |
| Trading Symbol                              | "CNA"                         |
| Security Exchange Name                      | CHX                           |

| X                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Boolean flag that is true when the XBRL content amends previously-filed or accepted submission.
+ References
No definition available.
+ Details**Name:**dei_AmendmentFlag**Namespace Prefix:**dei_**Data Type:**xbrli:booleanItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Area code of city
+ References
No definition available.
+ Details**Name:**dei_CityAreaCode**Namespace Prefix:**dei_**Data Type:**xbrli:normalizedStringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
For the EDGAR submission types of Form 8-K: the date of the report, the date of the earliest event reported; for the EDGAR submission types of Form N-1A: the filing date; for all other submission types: the end of the reporting or transition period.  The format of the date is YYYY-MM-DD.
+ References
No definition available.
+ Details**Name:**dei_DocumentPeriodEndDate**Namespace Prefix:**dei_**Data Type:**xbrli:dateItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The type of document being provided (such as 10-K, 10-Q, 485BPOS, etc). The document type is limited to the same value as the supporting SEC submission type, or the word 'Other'.
+ References
No definition available.
+ Details**Name:**dei_DocumentType**Namespace Prefix:**dei_**Data Type:**dei:submissionTypeItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Address Line 1 such as Attn, Building Name, Street Name
+ References
No definition available.
+ Details**Name:**dei_EntityAddressAddressLine1**Namespace Prefix:**dei_**Data Type:**xbrli:normalizedStringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Name of the City or Town
+ References
No definition available.
+ Details**Name:**dei_EntityAddressCityOrTown**Namespace Prefix:**dei_**Data Type:**xbrli:normalizedStringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Code for the postal or zip code
+ References
No definition available.
+ Details**Name:**dei_EntityAddressPostalZipCode**Namespace Prefix:**dei_**Data Type:**xbrli:normalizedStringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Name of the state or province.
+ References
No definition available.
+ Details**Name:**dei_EntityAddressStateOrProvince**Namespace Prefix:**dei_**Data Type:**dei:stateOrProvinceItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A unique 10-digit SEC-issued value to identify entities that have filed disclosures with the SEC. It is commonly abbreviated as CIK.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Exchange Act -Number 240 -Section 12 -Subsection b-2
+ Details**Name:**dei_EntityCentralIndexKey**Namespace Prefix:**dei_**Data Type:**dei:centralIndexKeyItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Indicate if registrant meets the emerging growth company criteria.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Exchange Act -Number 240 -Section 12 -Subsection b-2
+ Details**Name:**dei_EntityEmergingGrowthCompany**Namespace Prefix:**dei_**Data Type:**xbrli:booleanItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Commission file number. The field allows up to 17 characters. The prefix may contain 1-3 digits, the sequence number may contain 1-8 digits, the optional suffix may contain 1-4 characters, and the fields are separated with a hyphen.
+ References
No definition available.
+ Details**Name:**dei_EntityFileNumber**Namespace Prefix:**dei_**Data Type:**dei:fileNumberItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Two-character EDGAR code representing the state or country of incorporation.
+ References
No definition available.
+ Details**Name:**dei_EntityIncorporationStateCountryCode**Namespace Prefix:**dei_**Data Type:**dei:edgarStateCountryItemType**Balance Type:**na**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:**dei_EntityInformationLineItems**Namespace Prefix:**dei_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The exact name of the entity filing the report as specified in its charter, which is required by forms filed with the SEC.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Exchange Act -Number 240 -Section 12 -Subsection b-2
+ Details**Name:**dei_EntityRegistrantName**Namespace Prefix:**dei_**Data Type:**xbrli:normalizedStringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The Tax Identification Number (TIN), also known as an Employer Identification Number (EIN), is a unique 9-digit value assigned by the IRS.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Exchange Act -Number 240 -Section 12 -Subsection b-2
+ Details**Name:**dei_EntityTaxIdentificationNumber**Namespace Prefix:**dei_**Data Type:**dei:employerIdItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Local phone number for entity.
+ References
No definition available.
+ Details**Name:**dei_LocalPhoneNumber**Namespace Prefix:**dei_**Data Type:**xbrli:normalizedStringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Exchange Act -Number 240 -Section 13e -Subsection 4c
+ Details**Name:**dei_PreCommencementIssuerTenderOffer**Namespace Prefix:**dei_**Data Type:**xbrli:booleanItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Exchange Act -Number 240 -Section 14d -Subsection 2b
+ Details**Name:**dei_PreCommencementTenderOffer**Namespace Prefix:**dei_**Data Type:**xbrli:booleanItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Title of a 12(b) registered security.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Exchange Act -Number 240 -Section 12 -Subsection b
+ Details**Name:**dei_Security12bTitle**Namespace Prefix:**dei_**Data Type:**dei:securityTitleItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Name of the Exchange on which a security is registered.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Exchange Act -Number 240 -Section 12 -Subsection d1-1
+ Details**Name:**dei_SecurityExchangeName**Namespace Prefix:**dei_**Data Type:**dei:edgarExchangeCodeItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as soliciting material pursuant to Rule 14a-12 under the Exchange Act.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Exchange Act -Section 14a -Number 240 -Subsection 12
+ Details**Name:**dei_SolicitingMaterial**Namespace Prefix:**dei_**Data Type:**xbrli:booleanItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Trading symbol of an instrument as listed on an exchange.
+ References
No definition available.
+ Details**Name:**dei_TradingSymbol**Namespace Prefix:**dei_**Data Type:**dei:tradingSymbolItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as written communications pursuant to Rule 425 under the Securities Act.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Securities Act -Number 230 -Section 425
+ Details**Name:**dei_WrittenCommunications**Namespace Prefix:**dei_**Data Type:**xbrli:booleanItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**dei_EntityListingsExchangeAxis=exch_XNYS**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**dei_EntityListingsExchangeAxis=exch_XCHI**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |