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Commitments and Contingencies (Narrative) (Details) | Mar. 31, 2020 USD ($)
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Commitments and Contingencies Disclosure [Abstract] |
Unused loan commitments | $ 150,672,000
Standby letters of credit | 10,703,000
Commercial letters of credit | $ 652,000

X
- Definition Standby letters of credit outstanding. + References No definition available. + Details
Name: | cass_StandbyLettersOfCreditOutstanding
Namespace Prefix: | cass_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

X
- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Unused amount of commitments to extend credit to customers. + References No definition available. + Details
Name: | us-gaap_UnusedCommitmentsToExtendCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant