| Document and Entity Information | Jan. 31, 2020 |
| Cover [Abstract] | |
| Entity Registrant Name | CASS INFORMATION SYSTEMS INC |
| Amendment Flag | false |
| Entity Central Index Key | 0000708781 |
| Document Type | 8-K |
| Document Period End Date | Jan. 31, 2020 |
| Entity Incorporation State Country Code | MO |
| Entity File Number | 000-20827 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.50 per share |
| Trading Symbol | CASS |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
