Capital Requirements and Regulatory Restrictions (Schedule of Capital Amounts and Ratios) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cass Information Systems Inc. [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), actual amount	$ 234,389	$ 219,747
Common Equity Tier I Capital (to risk-weighted assets), actual amount	224,184	209,572
Tier I capital (to risk-weighted assets), actual amount	224,184	209,572
Tier I capital (to average assets), actual amount	$ 224,184	$ 209,572
Total capital (to risk-weighted assets), actual ratio	22.53%	22.75%
Common Equity Tier I Capital (to risk-weighted assets), actual ratio	21.55%	21.70%
Tier I capital (to risk-weighted assets), actual ratio	21.55%	21.70%
Tier I capital (to average assets), actual ratio	13.87%	13.83%
Total capital (to risk-weighted assets), capital requirements amount	$ 83,233	$ 77,272
Common Equity Tier I Capital (to risk-weighted assets), capital requirements amount	46,819	43,466
Tier I capital (to risk-weighted assets), capital requirements amount	62,425	57,954
Tier I capital (to average assets), capital requirements amount	$ 64,649	$ 60,620
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	$ 122,440	$ 110,576
Common Equity Tier I Capital (to risk-weighted assets), actual amount	114,603	102,769
Tier I capital (to risk-weighted assets), actual amount	114,603	102,769
Tier I capital (to average assets), actual amount	$ 114,603	$ 102,769
Total capital (to risk-weighted assets), actual ratio	17.01%	16.72%
Common Equity Tier I Capital (to risk-weighted assets), actual ratio	15.93%	15.54%
Tier I capital (to risk-weighted assets), actual ratio	15.93%	15.54%
Tier I capital (to average assets), actual ratio	14.99%	13.98%
Total capital (to risk-weighted assets), capital requirements amount	$ 57,568	$ 52,898
Common Equity Tier I Capital (to risk-weighted assets), capital requirements amount	32,382	29,755
Tier I capital (to risk-weighted assets), capital requirements amount	43,176	39,674
Tier I capital (to average assets), capital requirements amount	$ 30,581	$ 29,409
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	$ 71,960	$ 66,123
Common Equity Tier I Capital (to risk-weighted assets), requirement to be well capitalized amount	46,774	42,980
Tier I capital (to risk-weighted assets), requirement to be well capitalized amount	57,568	52,898
Tier I capital (to average assets), requirement to be well capitalized amount	$ 38,227	$ 36,761
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 Common Equity Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The Common Equity Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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Common Equity Tier 1 capital divided by risk weighted assets as defined by regulations.

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The amount of total capital as defined in the regulations.

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The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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

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The minimum amount of Tier 1 Leverage Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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