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Employee Benefit Plans - Schedule of Long-term Rate of Return on Assets (Details) | 12 Months Ended
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Dec. 31, 2023
Core Fixed Income |
Defined Benefit Plan Disclosure [Line Items] |
% of Total Portfolio | 86.00%
One-Year Nominal Return | 4.52%
Annual Standard Deviation | 4.30%
Global Equity |
Defined Benefit Plan Disclosure [Line Items] |
% of Total Portfolio | 14.00%
One-Year Nominal Return | 8.54%
Annual Standard Deviation | 18.12%

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- Definition The expected percentage of annual standard deviation on assets. + References No definition available. + Details
Name: | cass_AnnualStandardDeviation
Namespace Prefix: | cass_
Data Type: | dtr-types1:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition The expected percentage of one-year nominal return on assets. + References No definition available. + Details
Name: | cass_OneYearNominalReturn
Namespace Prefix: | cass_
Data Type: | dtr-types1:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition 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. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(i) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |