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Capital Requirements and Regulatory Restrictions (Schedule of Capital Amounts and Ratios) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Cass Information Systems Inc. [Member]
Total capital (to risk-weighted assets), actual amount	$ 207,468us-gaap_Capital / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember	$ 191,984us-gaap_Capital / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Tier I capital (to risk-weighted assets), actual amount	195,630us-gaap_TierOneRiskBasedCapital / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember	181,198us-gaap_TierOneRiskBasedCapital / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Tier I capital (to average assets), actual amount	195,630us-gaap_TierOneLeverageCapital / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember	181,198us-gaap_TierOneLeverageCapital / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Total capital (to risk-weighted assets), actual ratio	21.91%us-gaap_CapitalToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember	22.27%us-gaap_CapitalToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Tier I capital (to risk-weighted assets), actual ratio	20.66%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember	21.02%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Tier I capital (to average assets), actual ratio	13.42%us-gaap_TierOneLeverageCapitalToAverageAssets / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember	13.12%us-gaap_TierOneLeverageCapitalToAverageAssets / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Total capital (to risk-weighted assets), capital requirements amount	75,761us-gaap_CapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember	68,956us-gaap_CapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Tier I capital (to risk-weighted assets), capital requirements amount	37,880us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember	34,478us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Tier I capital (to average assets), capital requirements amount	43,742us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember	41,438us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Total capital (to risk-weighted assets), capital requirements ratio	8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember	8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Tier I capital (to risk-weighted assets), capital requirements ratio	4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember	4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Tier I capital (to average assets), capital requirements ratio	3.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember	3.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Cass Commercial Bank [Member]
Total capital (to risk-weighted assets), actual amount	91,249us-gaap_Capital / dei_LegalEntityAxis = cass_CassCommercialBankMember	83,168us-gaap_Capital / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to risk-weighted assets), actual amount	84,049us-gaap_TierOneRiskBasedCapital / dei_LegalEntityAxis = cass_CassCommercialBankMember	76,395us-gaap_TierOneRiskBasedCapital / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to average assets), actual amount	84,049us-gaap_TierOneLeverageCapital / dei_LegalEntityAxis = cass_CassCommercialBankMember	76,395us-gaap_TierOneLeverageCapital / dei_LegalEntityAxis = cass_CassCommercialBankMember
Total capital (to risk-weighted assets), actual ratio	15.88%us-gaap_CapitalToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember	15.38%us-gaap_CapitalToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to risk-weighted assets), actual ratio	14.62%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember	14.13%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to average assets), actual ratio	11.94%us-gaap_TierOneLeverageCapitalToAverageAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember	11.37%us-gaap_TierOneLeverageCapitalToAverageAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember
Total capital (to risk-weighted assets), capital requirements amount	45,977us-gaap_CapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassCommercialBankMember	43,256us-gaap_CapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to risk-weighted assets), capital requirements amount	22,989us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassCommercialBankMember	21,628us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to average assets), capital requirements amount	21,124us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassCommercialBankMember	20,162us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassCommercialBankMember
Total capital (to risk-weighted assets), capital requirements ratio	8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember	8.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to risk-weighted assets), capital requirements ratio	4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember	4.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to average assets), capital requirements ratio	3.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember	3.00%us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember
Total capital (to risk-weighted assets), requirement to be well capitalized amount	57,472us-gaap_CapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = cass_CassCommercialBankMember	54,071us-gaap_CapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to risk-weighted assets), requirement to be well capitalized amount	34,483us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = cass_CassCommercialBankMember	32,442us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to average assets), requirement to be well capitalized amount	$ 35,207us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = cass_CassCommercialBankMember	$ 33,603us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = cass_CassCommercialBankMember
Total capital (to risk-weighted assets), requirement to be well capitalized ratio	10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember	10.00%us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to risk-weighted assets), requirement to be well capitalized ratio	6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember	6.00%us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to average assets), requirement to be well capitalized ratio	5.00%us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember	5.00%us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets / dei_LegalEntityAxis = cass_CassCommercialBankMember

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The amount of total capital 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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