DOCUMENT AND ENTITY INFORMATION (USD $)	12 Months Ended
	Dec. 31, 2012	Mar. 05, 2013	Jun. 30, 2012
Entity Registrant Name	CASS INFORMATION SYSTEMS INC
Entity Central Index Key	0000708781
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Trading Symbol	cass
Entity Common Stock, Shares Outstanding		11,463,831
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2012
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2012
Entity Well-Known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 421,100,000