Cover Page - USD ($)	12 Months Ended
	Dec. 31, 2021	Feb. 15, 2022	Jun. 30, 2021
Cover [Abstract]
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	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 $0.50 per share
Trading Symbol	CASS
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 543,951,000
Entity Common Stock, Shares Outstanding		13,694,489
Documents Incorporated by Reference	Certain information required for Part III of this report is incorporated by reference to the Registrant’s Proxy Statement for the 2022 Annual Meeting of Shareholders.
Entity Central Index Key	0000708781
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY