| Cover Page Cover Page | Aug. 03, 2020 |
| Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Aug. 3, 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 |
