| **Cover Page Cover Page**                   | Feb. 10, 2020                 |
|---------------------------------------------|-------------------------------|
| **Entity Information [Line Items]**         |                          |
| Document Type                               | 8-K                           |
| Document Period End Date                    | Feb. 10,  2020                |
| 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                           |

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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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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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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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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.
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