Capital Requirements and Regulatory Restrictions - Schedule of Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Cass Information Systems, Inc.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), actual, amount	$ 269,580	$ 257,313
Common equity tier I capital (to risk-weighted assets), actual, amount	256,359	243,774
Tier I capital (to risk-weighted assets), actual, amount	256,359	243,774
Tier I capital (to average assets), actual, amount	$ 256,359	$ 243,774
Total capital (to risk-weighted assets), actual, ratio	15.49%	13.52%
Common equity tier I capital (to risk-weighted assets), actual, ratio	14.73%	12.80%
Tier I capital (to risk-weighted assets), actual, ratio	14.73%	12.80%
Tier I capital (to average assets), actual, ratio	10.71%	9.52%
Total capital (to risk-weighted assets), capital requirements, amount	$ 139,266	$ 152,306
Common equity tier I capital (to risk-weighted assets), capital requirements, amount	78,337	85,672
Tier I capital (to risk-weighted assets), capital requirements, amount	104,449	114,229
Tier I capital (to average assets), capital requirements, amount	$ 95,760	$ 102,386
Total capital (to risk-weighted assets), capital requirements, ratio	8.00%	8.00%
Common equity tier I capital (to risk-weighted assets), capital requirements, ratio	4.50%	4.50%
Tier I capital (to risk-weighted assets), capital requirements, ratio	6.00%	6.00%
Tier I capital (to average assets), capital requirements, ratio	4.00%	4.00%
Cass Commercial Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), actual, amount	$ 204,584	$ 186,075
Common equity tier I capital (to risk-weighted assets), actual, amount	192,104	172,848
Tier I capital (to risk-weighted assets), actual, amount	192,104	172,848
Tier I capital (to average assets), actual, amount	$ 192,104	$ 172,848
Total capital (to risk-weighted assets), actual, ratio	19.04%	16.00%
Common equity tier I capital (to risk-weighted assets), actual, ratio	17.88%	14.86%
Tier I capital (to risk-weighted assets), actual, ratio	17.88%	14.86%
Tier I capital (to average assets), actual, ratio	12.49%	10.77%
Total capital (to risk-weighted assets), capital requirements, amount	$ 85,964	$ 93,044
Common equity tier I capital (to risk-weighted assets), capital requirements, amount	48,355	52,337
Tier I capital (to risk-weighted assets), capital requirements, amount	64,473	69,783
Tier I capital (to average assets), capital requirements, amount	$ 61,526	$ 64,196
Total capital (to risk-weighted assets), capital requirements, ratio	8.00%	8.00%
Common equity tier I capital (to risk-weighted assets), capital requirements, ratio	4.50%	4.50%
Tier I capital (to risk-weighted assets), capital requirements, ratio	6.00%	6.00%
Tier I capital (to average assets), capital requirements, ratio	4.00%	4.00%
Total capital (to risk weighted assets), requirement to be well-capitalized, amount	$ 107,455	$ 116,305
Common equity tier I capital (to risk-weighted assets), requirement to be well-capitalized, amount	69,846	75,598
Tier I capital (to risk-weighted assets), requirement to be well-capitalized, amount	85,964	93,044
Tier I capital (to average assets), requirement to be well-capitalized, amount	$ 76,908	$ 80,245
Total capital (to risk weighted assets), requirement to be well-capitalized, ratio	10.00%	10.00%
Common equity tier I capital (to risk-weighted assets), requirement to be well-capitalized, ratio	6.50%	6.50%
Tier I capital (to risk-weighted assets), requirement to be well-capitalized, ratio	8.00%	8.00%
Tier I capital (to average assets), requirement to be well-capitalized, ratio	5.00%	5.00%