| **Cover Page Cover Page - USD ($) $ in Millions**   | 12 Months Ended                                                                                                                                                                                              |             |         |
|-----------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------|---------|
||
| **Entity Information [Line Items]**                 |                                                                                                                                                                                                         |        |    |
| Document Type                                       | 10-K                                                                                                                                                                                                         |        |    |
| Document Annual Report                              | true                                                                                                                                                                                                         |        |    |
| Document Period End Date                            | Dec. 31,  2019                                                                                                                                                                                               |        |    |
| 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 Well-known Seasoned Issuer                   | Yes                                                                                                                                                                                                          |        |    |
| Entity Voluntary Filers                             | No                                                                                                                                                                                                           |        |    |
| 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) |                                                                                                                                                                                                         | 271,412,591 |    |
| Entity Public Float                                 |                                                                                                                                                                                                         |        | $ 1,338 |
| Documents Incorporated by Reference                 | Portions of the CNA Financial Corporation Proxy Statement prepared for the 2020 annual meeting of shareholders, pursuant to Regulation 14A, are incorporated by reference into Part III of this report. |        |    |
| Entity Central Index Key                            | 0000021175                                                                                                                                                                                                   |        |    |
| Current Fiscal Year End Date                        | --12-31                                                                                                                                                                                                      |        |    |
| Document Fiscal Year Focus                          | 2019                                                                                                                                                                                                         |        |    |
| Document Fiscal Period Focus                        | FY                                                                                                                                                                                                           |        |    |
| 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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Documents incorporated by reference.
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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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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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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).
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The aggregate market value of the voting and non-voting common equity held by non-affiliates computed by reference to the price at which the common equity was last sold, or the average bid and asked price of such common equity, as of the last business day of the registrant's most recently completed second fiscal quarter.
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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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Indicate 'Yes' or 'No' if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Is used on Form Type: 10-K, 10-Q, 8-K, 20-F, 6-K, 10-K/A, 10-Q/A, 20-F/A, 6-K/A, N-CSR, N-Q, N-1A.
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