Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Oct. 25, 2019
Document and Entity Information [Abstract]    
Document Type 10-Q  
Document Period End Date Sep. 30, 2019  
Entity File Number 000-20827  
Entity Interactive Data Current Yes  
Entity Current Reporting Status Yes  
Entity Incorporation, State or Country Code MO  
Entity Address, City or Town St. Louis  
Entity Address, State or Province MO  
Entity Tax Identification Number 43-1265338  
Entity Address, Address Line One 12444 Powerscourt Drive, Suite 550  
Entity Address, Postal Zip Code 63131  
Local Phone Number 506-5500  
Entity Information, Former Legal or Registered Name N/A  
Amendment Flag false  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Entity Central Index Key 0000708781  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   14,497,763
City Area Code 314  
Document Quarterly Report true  
Document Transition Report false  
Title of 12(b) Security Common stock, par value $.50  
Trading Symbol CASS  
Name of Exchange on which Security is Registered NASDAQ