| **Cover Page Cover Page - shares**                  | 3 Months Ended                |             |
|-----------------------------------------------------|-------------------------------|-------------|
||
| **Entity Information [Line Items]**                 |                          |        |
| Document Type                                       | 10-Q                          |        |
| Document Quarterly Report                           | true                          |        |
| Document Period End Date                            | Mar. 31,  2020                |        |
| Document Transition Report                          | false                         |        |
| Entity File Number                                  | 1-5823                        |        |
| Entity Registrant Name                              | CNA FINANCIAL CORPORATION     |        |
| Entity Incorporation, State or Country Code         | DE                            |        |
| Entity Tax Identification Number                    | 36-6169860                    |        |
| Entity Address, Address Line One                    | 151 N. Franklin               |        |
| Entity Address, Postal Zip Code                     | 60606                         |        |
| Entity Address, City or Town                        | Chicago,                      |        |
| Entity Address, State or Province                   | IL                            |        |
| City Area Code                                      | 312                           |        |
| Local Phone Number                                  | 822-5000                      |        |
| Title of 12(b) Security                             | Common Stock, Par value $2.50 |        |
| Trading Symbol                                      | "CNA"                         |        |
| Entity Current Reporting Status                     | Yes                           |        |
| Entity Interactive Data Current                     | Yes                           |        |
| Entity Filer Category                               | Large Accelerated Filer       |        |
| Entity Small Business                               | false                         |        |
| Entity Emerging Growth Company                      | false                         |        |
| Entity Shell Company                                | false                         |        |
| Entity Common Stock, Shares Outstanding (in shares) |                          | 271,376,098 |
| Entity Central Index Key                            | 0000021175                    |        |
| Current Fiscal Year End Date                        | --03-31                       |        |
| Document Fiscal Year Focus                          | 2020                          |        |
| Document Fiscal Period Focus                        | Q1                            |        |
| Amendment Flag                                      | false                         |        |
| New York Stock Exchange                             |                          |        |
| **Entity Information [Line Items]**                 |                          |        |
| Security Exchange Name                              | NYSE                          |        |
| Chicago Stock Exchange                              |                          |        |
| **Entity Information [Line Items]**                 |                          |        |
| 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
End date of current fiscal year in the format --MM-DD.
+ References
No definition available.
+ Details**Name:**dei_CurrentFiscalYearEndDate**Namespace Prefix:**dei_**Data Type:**xbrli:gMonthDayItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fiscal period values are FY, Q1, Q2, and Q3.  1st, 2nd and 3rd quarter 10-Q or 10-QT statements have value Q1, Q2, and Q3 respectively, with 10-K, 10-KT or other fiscal year statements having FY.
+ References
No definition available.
+ Details**Name:**dei_DocumentFiscalPeriodFocus**Namespace Prefix:**dei_**Data Type:**dei:fiscalPeriodItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This is focus fiscal year of the document report in CCYY format. For a 2006 annual report, which may also provide financial information from prior periods, fiscal 2006 should be given as the fiscal year focus. Example: 2006.
+ References
No definition available.
+ Details**Name:**dei_DocumentFiscalYearFocus**Namespace Prefix:**dei_**Data Type:**xbrli:gYearItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The end date of the period reflected on the cover page if a periodic report. For all other reports and registration statements containing historical data, it is the date up through which that historical data is presented.  If there is no historical data in the report, use the filing date. The format of the date is CCYY-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
Boolean flag that is true only for a form used as an quarterly report.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form 10-Q -Number 240 -Section 13 -Subsection a-13
+ Details**Name:**dei_DocumentQuarterlyReport**Namespace Prefix:**dei_**Data Type:**xbrli:booleanItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Boolean flag that is true only for a form used as a transition report.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Forms 10-K, 10-Q, 20-F -Number 240 -Section 13 -Subsection a-1-
+ Details**Name:**dei_DocumentTransitionReport**Namespace Prefix:**dei_**Data Type:**xbrli:booleanItemType**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 Regulation 12B -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 number of shares or other units outstanding of each of registrant's classes of capital or common stock or other ownership interests, if and as stated on cover of related periodic report. Where multiple classes or units exist define each class/interest by adding class of stock items such as Common Class A [Member], Common Class B [Member] or Partnership Interest [Member] onto the Instrument [Domain] of the Entity Listings, Instrument.
+ References
No definition available.
+ Details**Name:**dei_EntityCommonStockSharesOutstanding**Namespace Prefix:**dei_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Indicate 'Yes' or 'No' whether registrants (1) have filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that registrants were required to file such reports), and (2) have been subject to such filing requirements for the past 90 days. This information should be based on the registrant's current or most recent filing containing the related disclosure.
+ References
No definition available.
+ Details**Name:**dei_EntityCurrentReportingStatus**Namespace Prefix:**dei_**Data Type:**dei:yesNoItemType**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 Regulation 12B -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
Indicate whether the registrant is one of the following: Large Accelerated Filer, Accelerated Filer, Non-accelerated Filer. Definitions of these categories are stated in Rule 12b-2 of the Exchange Act. This information should be based on the registrant's current or most recent filing containing the related disclosure.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation 12B -Number 240 -Section 12 -Subsection b-2
+ Details**Name:**dei_EntityFilerCategory**Namespace Prefix:**dei_**Data Type:**dei:filerCategoryItemType**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
Boolean flag that is true when the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or for such shorter period that the registrant was required to submit such files).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-T -Number 232 -Section 405
+ Details**Name:**dei_EntityInteractiveDataCurrent**Namespace Prefix:**dei_**Data Type:**dei:yesNoItemType**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 Regulation 12B -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
Boolean flag that is true when the registrant is a shell company as defined in Rule 12b-2 of the Exchange Act.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation 12B -Number 240 -Section 12 -Subsection b-2
+ Details**Name:**dei_EntityShellCompany**Namespace Prefix:**dei_**Data Type:**xbrli:booleanItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Indicates that the company is a Smaller Reporting Company (SRC).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation 12B -Number 240 -Section 12 -Subsection b-2
+ Details**Name:**dei_EntitySmallBusiness**Namespace Prefix:**dei_**Data Type:**xbrli:booleanItemType**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 Regulation 12B -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
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
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                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------|
| - 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:** |