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v3.20.1

Defined Pension Plans (Narrative) (Details) - USD ($) | 3 Months Ended
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Mar. 31, 2020 | Mar. 31, 2019
Defined Benefit Plan Disclosure [Line Items] |  |
Pension period | 30 years |
Defined Benefit Pension Plan [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Pension costs | $ 1,029,000 | $ 1,179,000
Unfunded Supplemental Executive Retirement Plan [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Pension costs | $ 145,000 | $ 195,000

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- Definition Defined Benefit Plan, Pension Period + References No definition available. + Details
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Namespace Prefix: | cass_
Data Type: | xbrli:durationItemType
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 Amount of cost (reversal of cost) for pension benefits. Excludes other postretirement benefits. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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
Balance Type: |
Period Type: |