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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2019
Feb. 19, 2020
Jun. 30, 2019
Document and Entity Information [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Transition Report false    
Document Period End Date Dec. 31, 2019    
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    
Name of Exchange on which Security is Registered NASDAQ    
Entity Well-Known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 702,445,000
Entity Common Stock, Shares Outstanding   14,552,402  
Documents Incorporated By Reference Text Block

Certain information required for Part III of this report is incorporated by reference to the Registrant’s Proxy Statement for the 2020 Annual Meeting of Shareholders.

   
Amendment Flag false    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Entity Central Index Key 0000708781    
Current Fiscal Year End Date --12-31