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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
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Cass Information Systems Inc. [Member] |  |
Total capital (to risk-weighted assets), actual amount | $ 207,468 us-gaap_Capital / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember | $ 191,984 us-gaap_Capital / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Tier I capital (to risk-weighted assets), actual amount | 195,630 us-gaap_TierOneRiskBasedCapital / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember | 181,198 us-gaap_TierOneRiskBasedCapital / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Tier I capital (to average assets), actual amount | 195,630 us-gaap_TierOneLeverageCapital / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember | 181,198 us-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 | 75,761 us-gaap_CapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember | 68,956 us-gaap_CapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Tier I capital (to risk-weighted assets), capital requirements amount | 37,880 us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember | 34,478 us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember
Tier I capital (to average assets), capital requirements amount | 43,742 us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassInformationSystemsIncMember | 41,438 us-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 | 91,249 us-gaap_Capital / dei_LegalEntityAxis = cass_CassCommercialBankMember | 83,168 us-gaap_Capital / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to risk-weighted assets), actual amount | 84,049 us-gaap_TierOneRiskBasedCapital / dei_LegalEntityAxis = cass_CassCommercialBankMember | 76,395 us-gaap_TierOneRiskBasedCapital / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to average assets), actual amount | 84,049 us-gaap_TierOneLeverageCapital / dei_LegalEntityAxis = cass_CassCommercialBankMember | 76,395 us-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 | 45,977 us-gaap_CapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassCommercialBankMember | 43,256 us-gaap_CapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to risk-weighted assets), capital requirements amount | 22,989 us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassCommercialBankMember | 21,628 us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to average assets), capital requirements amount | 21,124 us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacy / dei_LegalEntityAxis = cass_CassCommercialBankMember | 20,162 us-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 | 57,472 us-gaap_CapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = cass_CassCommercialBankMember | 54,071 us-gaap_CapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to risk-weighted assets), requirement to be well capitalized amount | 34,483 us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = cass_CassCommercialBankMember | 32,442 us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = cass_CassCommercialBankMember
Tier I capital (to average assets), requirement to be well capitalized amount | $ 35,207 us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized / dei_LegalEntityAxis = cass_CassCommercialBankMember | $ 33,603 us-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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- Definition The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958570-112826 + Details
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- Definition The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition Total Capital divided by risk weighted assets as defined in the regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition Tier 1 Risk Based Capital as defined in the regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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- Definition The minimum 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958570-112826 + Details
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- Definition The 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826 + Details
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