Capital Requirements and Regulatory Restrictions (Schedule of Capital Amounts and Ratios) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cass Information Systems Inc. [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), actual amount	$ 244,660	$ 234,389
Common Equity Tier I Capital (to risk-weighted assets), actual amount	234,435	224,184
Tier I capital (to risk-weighted assets), actual amount	234,435	224,184
Tier I capital (to average assets), actual amount	$ 234,435	$ 224,184
Total capital (to risk-weighted assets), actual ratio	21.38%	22.53%
Common Equity Tier I Capital (to risk-weighted assets), actual ratio	20.49%	21.55%
Tier I capital (to risk-weighted assets), actual ratio	20.49%	21.55%
Tier I capital (to average assets), actual ratio	13.89%	13.87%
Total capital (to risk-weighted assets), capital requirements amount	$ 91,550	$ 83,233
Common Equity Tier I Capital (to risk-weighted assets), capital requirements amount	51,497	46,819
Tier I capital (to risk-weighted assets), capital requirements amount	68,662	62,425
Tier I capital (to average assets), capital requirements amount	$ 67,500	$ 64,649
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	$ 137,894	$ 122,440
Common Equity Tier I Capital (to risk-weighted assets), actual amount	130,037	114,603
Tier I capital (to risk-weighted assets), actual amount	130,037	114,603
Tier I capital (to average assets), actual amount	$ 130,037	$ 114,603
Total capital (to risk-weighted assets), actual ratio	18.31%	17.01%
Common Equity Tier I Capital (to risk-weighted assets), actual ratio	17.26%	15.93%
Tier I capital (to risk-weighted assets), actual ratio	17.26%	15.93%
Tier I capital (to average assets), actual ratio	15.35%	14.99%
Total capital (to risk-weighted assets), capital requirements amount	$ 60,257	$ 57,568
Common Equity Tier I Capital (to risk-weighted assets), capital requirements amount	33,895	32,382
Tier I capital (to risk-weighted assets), capital requirements amount	45,193	43,176
Tier I capital (to average assets), capital requirements amount	$ 33,884	$ 30,581
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	$ 75,321	$ 71,960
Common Equity Tier I Capital (to risk-weighted assets), requirement to be well capitalized amount	48,959	46,774
Tier I capital (to risk-weighted assets), requirement to be well capitalized amount	60,257	57,568
Tier I capital (to average assets), requirement to be well capitalized amount	$ 42,354	$ 38,227
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 amount of Total Capital required to be categorized as well as capitalized 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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