DOCUMENT AND ENTITY INFORMATION - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Mar. 01, 2017	Jun. 30, 2016
Document and Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	FY
Entity Registrant Name	CASS INFORMATION SYSTEMS INC
Entity Central Index Key	0000708781
Trading Symbol	cass
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		11,197,226
Entity Well-Known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 548,000,000