Cover Page Cover Page - shares	3 Months Ended
	Mar. 31, 2020	Apr. 30, 2020
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
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 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)		271,376,098
Entity Central Index Key	0000021175
Current Fiscal Year End Date	--03-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
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