Capital Requirements and Regulatory Restrictions - Schedule of Capital Amounts and Ratios (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Cass Information Systems, Inc.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), actual, amount	$ 257,313	$ 240,265
Common equity tier I capital (to risk-weighted assets), actual, amount	243,774	228,224
Tier I capital (to risk-weighted assets), actual, amount	243,774	228,224
Tier I capital (to average assets), actual, amount	$ 243,774	$ 228,224
Total capital (to risk-weighted assets), actual, ratio	13.52%	14.86%
Common equity tier I capital (to risk-weighted assets), actual, ratio	12.80%	14.11%
Tier I capital (to risk-weighted assets), actual, ratio	12.80%	14.11%
Tier I capital (to average assets), actual, ratio	9.52%	9.21%
Total capital (to risk-weighted assets), capital requirements, amount	$ 152,306	$ 129,339
Common equity tier I capital (to risk-weighted assets), capital requirements, amount	85,672	72,764
Tier I capital (to risk-weighted assets), capital requirements, amount	114,229	97,019
Tier I capital (to average assets), capital requirements, amount	$ 102,386	$ 99,163
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	$ 186,075	$ 174,614
Common equity tier I capital (to risk-weighted assets), actual, amount	172,848	163,030
Tier I capital (to risk-weighted assets), actual, amount	172,848	163,030
Tier I capital (to average assets), actual, amount	$ 172,848	$ 163,030
Total capital (to risk-weighted assets), actual, ratio	16.00%	17.21%
Common equity tier I capital (to risk-weighted assets), actual, ratio	14.86%	16.07%
Tier I capital (to risk-weighted assets), actual, ratio	14.86%	16.07%
Tier I capital (to average assets), actual, ratio	10.77%	11.05%
Total capital (to risk-weighted assets), capital requirements, amount	$ 93,044	$ 81,163
Common equity tier I capital (to risk-weighted assets), capital requirements, amount	52,337	45,654
Tier I capital (to risk-weighted assets), capital requirements, amount	69,783	60,872
Tier I capital (to average assets), capital requirements, amount	$ 64,196	$ 59,036
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	$ 116,305	$ 101,454
Common equity tier I capital (to risk-weighted assets), requirement to be well-capitalized, amount	75,598	65,945
Tier I capital (to risk-weighted assets), requirement to be well-capitalized, amount	93,044	81,163
Tier I capital (to average assets), requirement to be well-capitalized, amount	$ 80,245	$ 73,795
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%