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Cover Page - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Feb. 06, 2026
Jun. 30, 2025
Entity Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2025    
Current Fiscal Year End Date --12-31    
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    
Document Financial Statement Error Correction [Flag] false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding (in shares)   270,673,371  
Entity Public Float     $ 998
Documents Incorporated by Reference
DOCUMENTS INCORPORATED BY REFERENCE
Portions of the CNA Financial Corporation Proxy Statement prepared for the 2026 annual meeting of shareholders, pursuant to Regulation 14A, are incorporated by reference into Part III of this report.
    
Entity Central Index Key 0000021175    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Amendment Flag false    
New York Stock Exchange      
Entity Information [Line Items]      
Security Exchange Name NYSE    
NYSE Texas      
Entity Information [Line Items]      
Security Exchange Name CHX