Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Feb. 04, 2022	Jun. 30, 2021
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
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
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding (in shares)		271,365,221
Entity Public Float			$ 1,255
Documents Incorporated by Reference	Portions of the CNA Financial Corporation Proxy Statement prepared for the 2022 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	2021
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