Capital Requirements and Regulatory Restrictions (Schedule of Capital Amounts and Ratios) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Cass Information Systems Inc. [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), actual amount	$ 249,954	$ 244,660
Common Equity Tier I Capital (to risk-weighted assets), actual amount	239,398	234,435
Tier I capital (to risk-weighted assets), actual amount	239,398	234,435
Tier I capital (to average assets), actual amount	$ 239,398	$ 234,435
Total capital (to risk-weighted assets), actual ratio	19.70%	21.38%
Common Equity Tier I Capital (to risk-weighted assets), actual ratio	18.86%	20.49%
Tier I capital (to risk-weighted assets), actual ratio	18.86%	20.49%
Tier I capital (to average assets), actual ratio	13.24%	13.89%
Total capital (to risk-weighted assets), capital requirements amount	$ 101,530	$ 91,550
Common Equity Tier I Capital (to risk-weighted assets), capital requirements amount	57,110	51,497
Tier I capital (to risk-weighted assets), capital requirements amount	76,147	68,662
Tier I capital (to average assets), capital requirements amount	$ 72,329	$ 67,500
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 [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), actual amount	$ 154,011	$ 137,894
Common Equity Tier I Capital (to risk-weighted assets), actual amount	145,673	130,037
Tier I capital (to risk-weighted assets), actual amount	145,673	130,037
Tier I capital (to average assets), actual amount	$ 145,673	$ 130,037
Total capital (to risk-weighted assets), actual ratio	19.32%	18.31%
Common Equity Tier I Capital (to risk-weighted assets), actual ratio	18.27%	17.26%
Tier I capital (to risk-weighted assets), actual ratio	18.27%	17.26%
Tier I capital (to average assets), actual ratio	16.64%	15.35%
Total capital (to risk-weighted assets), capital requirements amount	$ 63,778	$ 60,257
Common Equity Tier I Capital (to risk-weighted assets), capital requirements amount	35,875	33,895
Tier I capital (to risk-weighted assets), capital requirements amount	47,833	45,193
Tier I capital (to average assets), capital requirements amount	$ 35,012	$ 33,884
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	$ 79,722	$ 75,321
Common Equity Tier I Capital (to risk-weighted assets), requirement to be well capitalized amount	51,819	48,959
Tier I capital (to risk-weighted assets), requirement to be well capitalized amount	63,778	60,257
Tier I capital (to average assets), requirement to be well capitalized amount	$ 43,765	$ 42,354
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%

Common Equity Tier 1 Risk Based Capital as defined in the regulations.

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The minimum Common Equity Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The amount of total capital as defined in the regulations.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

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Tier 1 Risk Based Capital as defined in the regulations.

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