Capital Requirements and Regulatory Restrictions (Schedule of Capital Amounts and Ratios) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Cass Information Systems Inc. [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), actual amount	$ 219,747	$ 212,717
Common Equity Tier I Capital (to risk-weighted assets), actual amount	209,572	201,312
Tier I capital (to risk-weighted assets), actual amount	209,572	201,312
Tier I capital (to average assets), actual amount	$ 209,572	$ 201,312
Total capital (to risk-weighted assets), actual ratio	22.75%	23.31%
Common Equity Tier I Capital (to risk-weighted assets), actual ratio	21.70%	22.06%
Tier I capital (to risk-weighted assets), actual ratio	21.70%	22.06%
Tier I capital (to average assets), actual ratio	13.83%	13.88%
Total capital (to risk-weighted assets), capital requirements amount	$ 77,272	$ 72,994
Common Equity Tier I Capital (to risk-weighted assets), capital requirements amount	43,466	41,059
Tier I capital (to risk-weighted assets), capital requirements amount	57,954	54,746
Tier I capital (to average assets), capital requirements amount	$ 60,620	$ 43,496
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%	3.00%
Cass Commercial Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), actual amount	$ 110,576	$ 99,872
Common Equity Tier I Capital (to risk-weighted assets), actual amount	102,769	92,470
Tier I capital (to risk-weighted assets), actual amount	102,769	92,470
Tier I capital (to average assets), actual amount	$ 102,769	$ 92,470
Total capital (to risk-weighted assets), actual ratio	16.72%	16.90%
Common Equity Tier I Capital (to risk-weighted assets), actual ratio	15.54%	15.65%
Tier I capital (to risk-weighted assets), actual ratio	15.54%	15.65%
Tier I capital (to average assets), actual ratio	13.98%	13.15%
Total capital (to risk-weighted assets), capital requirements amount	$ 52,898	$ 47,281
Common Equity Tier I Capital (to risk-weighted assets), capital requirements amount	29,755	26,596
Tier I capital (to risk-weighted assets), capital requirements amount	39,674	35,461
Tier I capital (to average assets), capital requirements amount	$ 29,409	$ 21,093
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%	3.00%
Total capital (to risk-weighted assets), requirement to be well capitalized amount	$ 66,123	$ 59,102
Common Equity Tier I Capital (to risk-weighted assets), requirement to be well capitalized amount	42,980	38,416
Tier I capital (to risk-weighted assets), requirement to be well capitalized amount	52,898	47,281
Tier I capital (to average assets), requirement to be well capitalized amount	$ 36,761	$ 35,155
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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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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