| **Cover Page Cover Page - shares**                  | 6 Months Ended                |             |
|-----------------------------------------------------|-------------------------------|-------------|
||
| **Entity Information [Line Items]**                 |                          |        |
| Document Type                                       | 10-Q                          |        |
| Document Quarterly Report                           | true                          |        |
| Document Period End Date                            | Jun. 30,  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) |                          | 271,381,340 |
| Entity Central Index Key                            | 0000021175                    |        |
| Current Fiscal Year End Date                        | --12-31                       |        |
| Document Fiscal Year Focus                          | 2020                          |        |
| Document Fiscal Period Focus                        | Q2                            |        |
| 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                           |        |

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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.
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Name of the City or Town
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Name of the state or province.
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A unique 10-digit SEC-issued value to identify entities that have filed disclosures with the SEC. It is commonly abbreviated as CIK.
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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.
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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.
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Indicate if registrant meets the emerging growth company criteria.
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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.
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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.
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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.
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The exact name of the entity filing the report as specified in its charter, which is required by forms filed with the SEC.
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Boolean flag that is true when the registrant is a shell company as defined in Rule 12b-2 of the Exchange Act.
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Indicates that the company is a Smaller Reporting Company (SRC).
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The Tax Identification Number (TIN), also known as an Employer Identification Number (EIN), is a unique 9-digit value assigned by the IRS.
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Local phone number for entity.
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Title of a 12(b) registered security.
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