Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	Apr. 22, 2021
Document and Entity Information [Abstract]
Entity Central Index Key	0000708781
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Amendment Flag	false
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Mar. 31, 2021
Entity File Number	000-20827
Entity Registrant Name	CASS INFORMATION SYSTEMS, INC.
Entity Incorporation, State or Country Code	MO
Entity Tax Identification Number	43-1265338
Entity Address, Address Line One	12444 Powerscourt Drive
Entity Address, Address Line Two	Suite 550
Entity Address, City or Town	St. Louis
Entity Address, State or Province	MO
Entity Address, Postal Zip Code	63131
City Area Code	314
Local Phone Number	506-5500
Title of 12(b) Security	Common stock, par value $.50
Trading Symbol	CASS
Name of Exchange on which Security is Registered	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		14,400,909