Capital Requirements and Regulatory Restrictions (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Cass Information Systems, Inc. [Member]
Total capital (to risk-weighted assets), actual amount	$ 175,802	$ 166,605
Tier I capital (to risk-weighted assets), actual amount	164,729	155,638
Tier I capital (to average assets), actual amount	164,729	155,638
Total capital (to risk-weighted assets), actual ratio	19.87%	19.03%
Tier I capital (to risk-weighted assets), actual ratio	18.62%	17.78%
Tier I capital (to average assets), actual ratio	12.26%	11.53%
Total capital (to risk-weighted assets), capital requirements amount	70,767	70,033
Tier I capital (to risk-weighted assets), capital requirements amount	35,384	35,016
Tier I capital (to average assets), capital requirements amount	40,294	40,502
Total capital (to risk-weighted assets), capital requirements ratio	8.00%	8.00%
Tier I capital (to risk-weighted assets), capital requirements ratio	4.00%	4.00%
Tier I capital (to average assets), capital requirements ratio	3.00%	3.00%
Total capital (to risk-weighted assets), requirement to be well capitalized amount	0	0
Tier I capital (to risk-weighted assets), requirement to be well capitalized amount	0	0
Tier I capital (to average assets), requirement to be well capitalized amount	0	0
Total capital (to risk-weighted assets), requirement to be well capitalized ratio	0.00%	0.00%
Tier I capital (to risk-weighted assets), requirement to be well capitalized ratio	0.00%	0.00%
Tier I capital (to average assets), requirement to be well capitalized ratio	0.00%	0.00%
Cass Commercial Bank [Member]
Total capital (to risk-weighted assets), actual amount	75,300	66,851
Tier I capital (to risk-weighted assets), actual amount	68,261	60,248
Tier I capital (to average assets), actual amount	68,261	60,248
Total capital (to risk-weighted assets), actual ratio	13.41%	12.71%
Tier I capital (to risk-weighted assets), actual ratio	12.16%	11.46%
Tier I capital (to average assets), actual ratio	10.64%	9.49%
Total capital (to risk-weighted assets), capital requirements amount	44,909	42,070
Tier I capital (to risk-weighted assets), capital requirements amount	22,454	21,035
Tier I capital (to average assets), capital requirements amount	19,238	19,044
Total capital (to risk-weighted assets), capital requirements ratio	8.00%	8.00%
Tier I capital (to risk-weighted assets), capital requirements ratio	4.00%	4.00%
Tier I capital (to average assets), capital requirements ratio	3.00%	3.00%
Total capital (to risk-weighted assets), requirement to be well capitalized amount	56,136	52,588
Tier I capital (to risk-weighted assets), requirement to be well capitalized amount	33,682	31,553
Tier I capital (to average assets), requirement to be well capitalized amount	$ 32,063	$ 31,741
Total capital (to risk-weighted assets), requirement to be well capitalized ratio	10.00%	10.00%
Tier I capital (to risk-weighted assets), requirement to be well capitalized ratio	6.00%	6.00%
Tier I capital (to average assets), requirement to be well capitalized ratio	5.00%	5.00%