Capital Requirements and Regulatory Restrictions (Schedule of Capital Amounts and Ratios) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Cass Information Systems Inc. [Member]
Total capital (to risk-weighted assets), actual amount	$ 212,717	$ 207,468
Common Equity Tier I Capital (to risk-weighted assets), actual amount	201,312	195,630
Tier I capital (to risk-weighted assets), actual amount	201,312	195,630
Tier I capital (to average assets), actual amount	$ 201,312	$ 195,630
Total capital (to risk-weighted assets), actual ratio	23.31%	21.91%
Common Equity Tier I Capital (to risk-weighted assets), actual ratio	22.06%	20.66%
Tier I capital (to risk-weighted assets), actual ratio	22.06%	20.66%
Tier I capital (to average assets), actual ratio	13.88%	13.42%
Total capital (to risk-weighted assets), capital requirements amount	$ 72,994	$ 75,761
Common Equity Tier I Capital (to risk-weighted assets), capital requirements amount	41,059	37,880
Tier I capital (to risk-weighted assets), capital requirements amount	54,746	37,880
Tier I capital (to average assets), capital requirements amount	$ 43,496	$ 43,742
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%	4.00%
Tier I capital (to risk-weighted assets), capital requirements ratio	6.00%	4.00%
Tier I capital (to average assets), capital requirements ratio	3.00%	3.00%
Cass Commercial Bank [Member]
Total capital (to risk-weighted assets), actual amount	$ 99,872	$ 91,249
Common Equity Tier I Capital (to risk-weighted assets), actual amount	92,470	84,049
Tier I capital (to risk-weighted assets), actual amount	92,470	84,049
Tier I capital (to average assets), actual amount	$ 92,470	$ 84,049
Total capital (to risk-weighted assets), actual ratio	16.90%	15.88%
Common Equity Tier I Capital (to risk-weighted assets), actual ratio	15.65%	14.62%
Tier I capital (to risk-weighted assets), actual ratio	15.65%	14.62%
Tier I capital (to average assets), actual ratio	13.15%	11.94%
Total capital (to risk-weighted assets), capital requirements amount	$ 47,281	$ 45,977
Common Equity Tier I Capital (to risk-weighted assets), capital requirements amount	26,596	22,989
Tier I capital (to risk-weighted assets), capital requirements amount	35,461	22,989
Tier I capital (to average assets), capital requirements amount	$ 21,093	$ 21,124
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%	4.00%
Tier I capital (to risk-weighted assets), capital requirements ratio	6.00%	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	$ 59,102	$ 57,472
Common Equity Tier I Capital (to risk-weighted assets), requirement to be well capitalized amount	38,416	34,483
Tier I capital (to risk-weighted assets), requirement to be well capitalized amount	47,281	34,483
Tier I capital (to average assets), requirement to be well capitalized amount	$ 35,155	$ 35,207
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%	6.00%
Tier I capital (to risk-weighted assets), requirement to be well capitalized ratio	8.00%	6.00%
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 amount of Common Equity Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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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 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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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 for capital adequacy purposes divided by average assets as defined in the regulations.

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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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