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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, 2017
Fixed Income [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Percent of Total Portfolio | 51.00%
One-Year Nominal Return | 4.55%
Annual Standard Deviation | 4.55%
Large Cap U.S. Equities [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Percent of Total Portfolio | 14.00%
One-Year Nominal Return | 7.00%
Annual Standard Deviation | 15.85%
Small Cap U.S. Equities [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Percent of Total Portfolio | 5.00%
One-Year Nominal Return | 8.10%
Annual Standard Deviation | 19.70%
International (Developed) [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Percent of Total Portfolio | 25.00%
One-Year Nominal Return | 7.99%
Annual Standard Deviation | 17.93%
International (Emerging) [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Percent of Total Portfolio | 5.00%
One-Year Nominal Return | 10.56%
Annual Standard Deviation | 27.35%

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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: | num: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: | num: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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
Data Type: | na
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- Details
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Namespace Prefix: |
Data Type: | na
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
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