Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2016	Apr. 28, 2016
Document and Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q1
Entity Registrant Name	CASS INFORMATION SYSTEMS INC
Entity Central Index Key	0000708781
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		11,242,376